Difference between oracle database entities

Hi,

A database consists of several files (controlfile for database definition, datafiles to store data and online redo logs for crash recovery)
When you create objects you don't directly assign a datafile to store their data. You assign a tablespace, that is a group of datafiles, and the object's data will be spread across the tablespace datafiles. This is for manageability: tablespace can have specific storage attributes, they can be backeup, offlines, etc. as atomic parts of the database.
So, database is only storage on disk. But you need some processes to access to it.
Then you have a server attached to the disk, that run the oracle software as processes and shared memory: this is the instance.
You can have several servers (and then several instances) that access to the same database (then the same disks), and this is known as RAC.

Tablespace are data related with object data. But objects themselves are logically grouped in schema.
For example a table (owned by a schema) can have one or several segments to store its data, and those segments are stored in tablepaces. What that means is that the owner schema will define the table (columns, grants, etc.) and dat awill be store in some blocks within the tablespace datafiles.

Now, about the communication, let's do it quickly:
When data is needed, the instance processes read data from disk (datafiles) and put them in instance memory.
When data is updated, the instance processes update data in memory, records the changes to redo logs, and then - most of the time asynchronously, instance process writes updated data to disk.

So you see:
- how data is stored (database>tablespace>datafiles)
- how data is defined (database->schemas->objects)
- how data is accessed (server->instance->processes)
When user queries database objects, it communicates with a database server that maps the logical objects (in database dictionary) to physical data (on disk)  , and then reads/update the database datafiles

Regards,
Franck.

anandmahajan, you have uploaded copyrighted material !

Hmmm how should we delete it.

vittalmareddy, do you have any other questions?

Good answers were provided & your questions can be very easily researched via google. Those are database 101 concepts and are the subjects of many, many blogs, wikis, and documentation.