Each transaction on the database is independent of all others. So if I have a a connection, and you have a connection, then we can each commit and rollback without impacting each other.
Similarly, if I change some data (and have not yet committed), then you will not see the changes until I commit. Even further, if you start a query at 9am (that runs for 10mins), then if I change data at 9:02am *and* commit it, then even then, you will *still* not see the changed data in your query, because your query started at 9am, and thus sees a view of the data as it was at 9am.
Its one of the great things about relational databases.
It sounds like perhaps you are just getting started with databases, or Oracle. Have a read of the Concepts guide - it will be of great assistance
http://docs.oracle.com/database/121/CNCPT/toc.htm