sync local files with server files

Dear All

Scenario: I want to develop an application.The application should be able to connect to my remote server and download data to the local disk , while downloading it should check for new files and only download the new ones simultaneously creating the required(new) folders.

Problem: I have no idea how to compare the files in the server with the ones in the local disk.How to download only the new files from the server to the local disk?

What i am thinking?: I want to sync the files in the local machine with the ones in the server.and synchronize also the server files to local

thanks