what is aar in gradle

HI,
I have a following build.gradle file :

apply plugin: 'com.android.application'

dependencies {
    compile 'com.android.support:multidex:1.0.0'

    compile('com.android.support:support-v4:21.0.0') {
        transitive = true
    }
    compile 'com.android.support:appcompat-v7:19.1.0'
    compile 'com.android.support:support-annotations:+'
    compile project(':sip')
    compile project(':sip-contracts')
    compile project(':utils')
    compile project(':account')
    compile project(':calldata')
    compile project(':onboarding')
    compile project(':boiling')
    compile project(':commons')
    compile files('libs/MobileAppTracker-3.4.jar')
    compile('to.talk.aragorn:logging:0.1-SNAPSHOT@aar') {
        transitive = true
    }
    compile ('to.talk.aragorn:gcm-client:0.4-SNAPSHOT@aar'){
        transitive = true
    }
    compile 'to.talk.aragorn:network-utils:0.12-SNAPSHOT@aar'
    compile('to.talk.aragorn:kvstore:0.3-SNAPSHOT@aar') {
        transitive = true
    }
    compile('to.talk.aragorn:error-reporting:0.2-SNAPSHOT@aar') {
        transitive = true
    }
    compile 'to.talk.aragorn:log-reporting:0.2-SNAPSHOT@aar'
    compile('to.talk.aragorn:event-utils:0.1-SNAPSHOT@aar') {
        transitive = true
    }
    compile('to.talk.aragorn:exception-utils:0.1-SNAPSHOT@aar') {
        transitive = true
    }
    compile('to.talk.aragorn:sqlite-db:0.2-SNAPSHOT@aar') {
        transitive = true
    }
    compile 'to.talk.legolas:olympus-commons:0.32-SNAPSHOT@aar'
    compile 'to.talk.legolas:conman-client:1.03-SNAPSHOT@aar'

    compile 'to.talk.legolas:kuber-client:0.22-SNAPSHOT@aar'
    compile('to.talk.aragorn:phonebook:0.1-SNAPSHOT@aar') {
        transitive = true
    }
    compile('to.talk.legolas:stream-client:0.40-SNAPSHOT@aar') {
        transitive = true
    }
    compile('to.talk.legolas:zeus-client:0.4-SNAPSHOT@aar') {
        transitive = true
    }
    compile 'com.google.guava:guava:17.0'

    compile project(':contact')
    compile 'de.greenrobot:eventbus:2.2.1'
    compile 'to.talk.aragorn:commons:0.30-SNAPSHOT@aar'
    compile 'de.hdodenhof:circleimageview:1.1.1'
    compile 'com.madgag.spongycastle:core:1.50.0.0'
    compile 'com.google.android.gms:play-services-ads:6.5.+'
    compile('to.talk.legolas:fb-sdk:0.2-SNAPSHOT@aar')
}


def computeAppBuildDate() {
    def date = new Date()
    def formattedDate = date.format('yyyyMMddHHmmss')
    return "\"${formattedDate}\""
}

def getDomain() {
    return "kujo.app"
}

android {
    compileSdkVersion 21
    buildToolsVersion '21.1.1'
    sourceSets {
        main {
            manifest.srcFile 'AndroidManifest.xml'
            java.srcDirs = ['src']
            aidl.srcDirs = ['src']
            jniLibs.srcDirs = ['jniLibs']
            renderscript.srcDirs = ['src']
            res.srcDirs = ['res', 'res-flags']
            assets.srcDirs = ['assets']
        }
    }

    dexOptions {
        preDexLibraries = false
        javaMaxHeapSize "4g"
    }

    defaultConfig {
        minSdkVersion 15
        targetSdkVersion 19
        versionCode 11404
        versionName "1.14.04"
        buildConfigField "String", "SITE_URL", "\"http://www.tring.me\""
        buildConfigField "String", "TERMS_URL", "\"http://www.tring.me/terms.html\""
        buildConfigField "String", "DETAILED_STATS_REPORTING_ENDPOINT", "\"https://clog.handler.talk.to/tring_stats_detailed\""
        buildConfigField "String", "SUMMARY_STATS_REPORTING_ENDPOINT", "\"https://clog.handler.talk.to/tring_stats_summary\""
        buildConfigField "String", "FEEDBACK_STATS_REPORTING_ENDPOINT", "\"https://clog.handler.talk.to/tring_stats_feedback\""
        buildConfigField "String", "CALL_TERMINATION_REPORTING_ENDPOINT", "\"https://clog.handler.talk.to/tring_stats_hangup\""
        buildConfigField "String", "DOMAIN", "\"" + getDomain() + "\""
        buildConfigField "String", "CONMAN_ENDPOINT", "\"conman." + getDomain() + "\""
        buildConfigField "String", "KUBER_ENDPOINT", "\"kuber." + getDomain() + "\""
        buildConfigField "olympus.clients.commons.config.ClientType", "clientType", "olympus.clients.commons.config.ClientType.KUJO"
        buildConfigField "String", "ERROR_REPORTING_ENDPOINT", "\"error_reporting\""
        buildConfigField "String", "ANALYTICS_REPORTING_ENDPOINT", "\"analytics\""
        buildConfigField "String", "LOG_REPORTING_ENDPOINT", "\"https://clog-staging.handler.talk.to/rivadroid\""
        buildConfigField "boolean", "SEND_LOGS_BY_MAIL", "false"
        buildConfigField "boolean", "IS_LOG_STORAGE_ENABLED", "false"
        buildConfigField "String", "APOLLO_VERSION", "\"2.0\""
        buildConfigField "String", "SUPPORT_EMAIL_ADDRESS", "\"teamtringandroid@directi.com.go.to\""
        buildConfigField "String", "SIP_REGISTRAR_REALM", "\"mvoip\""
        buildConfigField "long", "CALL_STAT_REPORTING_TIME_IN_SEC", "300"
        buildConfigField "String", "MAT_ADVERTISER_ID", "\"22176\""
        buildConfigField "String", "MAT_CONVERSION_KEY", "\"d1b932ce5646b82c6183a15e8bc22a28\""
        buildConfigField "boolean", "IS_MAT_ENABLED", "false"
        buildConfigField "String", "UDP_ECHO_SERVER_ONE", "\"uecho1.use1b.talk.to\""
        buildConfigField "String", "UDP_ECHO_SERVER_TWO", "\"uecho1.use1d.talk.to\""
        buildConfigField "String", "UDP_ECHO_MESSAGE", "\"UECHO 1.0\""
        buildConfigField "String", "GCM_SENDER_ID", "\"244757587477\""
        buildConfigField "String", "CRITTERCISM_APP_ID", "\"54604de50729df3f38000001\""
        buildConfigField "boolean", "IS_FB_SDK_ENABLED", "false"
        buildConfigField "String", "FB_APP_ID", "\"291919911003322\""
        multiDexEnabled = true
    }

    lintOptions {
        abortOnError false
    }

    packagingOptions {
        exclude 'META-INF/LICENSE'
        exclude 'META-INF/DEPENDENCIES'
        exclude 'META-INF/NOTICE'
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }


    signingConfigs {
        release {
            storeFile file(TRING_SIGNING_CONFIG_STORE_FILE_NAME)
            storePassword TRING_SIGNING_CONFIG_STORE_PASSWORD
            keyAlias TRING_SIGNING_CONFIG_KEY_ALIAS
            keyPassword TRING_SIGNING_CONFIG_KEY_PASSWORD
        }
    }

    buildTypes {
        release {
            debuggable false
            minifyEnabled false
            proguardFile file('proguard-project.txt')
            proguardFile getDefaultProguardFile('proguard-android-optimize.txt')
            signingConfig signingConfigs.release
            buildConfigField "boolean", "ERROR_REPORTING", "true"
            buildConfigField "boolean", "LOG_REPORTING", "false"
            buildConfigField "boolean", "ANALYTICS_REPORTING", "true"
            buildConfigField "String", "APP_BUILD_DATE", computeAppBuildDate()
            buildConfigField "boolean", "SEND_LOGS_BY_MAIL", "true"
            buildConfigField "boolean", "IS_LOG_STORAGE_ENABLED", "true"
            buildConfigField "boolean", "IS_MAT_ENABLED", "true"
            buildConfigField "String", "SUPPORT_EMAIL_ADDRESS", "\"android@tring.me\""
            buildConfigField "boolean", "IS_FB_SDK_ENABLED", "true"
            buildConfigField "String", "FB_APP_ID", "\"291855387676441\""
        }

        debug {
            applicationIdSuffix ".debug"
            minifyEnabled false
            debuggable true
            buildConfigField "boolean", "ERROR_REPORTING", "false"
            buildConfigField "boolean", "LOG_REPORTING", "false"
            buildConfigField "boolean", "ANALYTICS_REPORTING", "false"
            buildConfigField "String", "APP_BUILD_DATE", computeAppBuildDate()
        }

        qa.initWith(buildTypes.debug)
        qa {
            applicationIdSuffix ".qa"
            buildConfigField "boolean", "ERROR_REPORTING", "true"
            buildConfigField "boolean", "SEND_LOGS_BY_MAIL", "true"
            buildConfigField "boolean", "IS_LOG_STORAGE_ENABLED", "true"
        }

        alpha.initWith(buildTypes.release)
        alpha {
            applicationIdSuffix ".alpha"
            minifyEnabled false
            debuggable true
            signingConfig signingConfigs.release
            buildConfigField "boolean", "IS_FB_SDK_ENABLED", "true"
            buildConfigField "boolean", "SEND_LOGS_BY_MAIL", "true"
            buildConfigField "boolean", "IS_LOG_STORAGE_ENABLED", "true"
            buildConfigField "String", "SUPPORT_EMAIL_ADDRESS", "\"teamtringandroid@directi.com.go.to\""
            buildConfigField "String", "FB_APP_ID", "\"291919911003322\""
        }
    }
    productFlavors {
        prod {
            buildConfigField "olympus.clients.commons.config.Environment", "environment", "olympus.clients.commons.config.Environment.PRODUCTION"
            buildConfigField "String", "SIP_REGISTRAR_IP", "\"209.15.246.86\""
            buildConfigField "int", "SIP_REGISTRAR_PORT", "4000"
            buildConfigField "String", "DOOR_HOST_NAME", "\"doormobile.handler.talk.to\""
            buildConfigField "String", "DOOR_PORT", "\"443\""

        }

        staging {
            buildConfigField "olympus.clients.commons.config.Environment", "environment", "olympus.clients.commons.config.Environment.STAGING"
            buildConfigField "String", "SIP_REGISTRAR_IP", "\"68.178.129.125\""
            buildConfigField "int", "SIP_REGISTRAR_PORT", "4000"
            buildConfigField "String", "DOOR_HOST_NAME", "\"doorstaging.handler.talk.to\""
            buildConfigField "String", "DOOR_PORT", "\"5222\""

        }
    }
}

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I am new to gradle. What doe the following line means :
   compile 'to.talk.aragorn:network-utils:0.12-SNAPSHOT@aar'

whats an aar ?

Thanks