You've received good answers from the other 3 people. It appears that you may be having a lot of Oracle questions in the future. If you have more questions you can contact me.
My own answer to your question would be:
Oracle Corporation is the second largest software company in the world, behind Microsoft. Their main product is the Oracle relational database management system (RDBMS). This is often referred to as the Oracle Server Software. This software runs on Windows and UNIX servers. It allows for the storage of data.
The Oracle RDBMS is robust (very stable) even under very large user and data loads. It is very scalable, so it can grow with your company. The data stored in the system can be queried directly using Structured Query Language (SQL) or by using a reporting tool such as Crystal Reports, business intelligence tools such as Cognos and Business Objects, Microsoft products such as Access or Excel, web technologies such as JDBC, and the list goes on.
Often Oracle data is "converted" or transferred from some older technology (legacy system) such as AS400. There are tools that allow data visibility and transfer between Oracle and Access, AS400, MS SQL Server, etc.
Many of the features/tools that I described are sold as add-ons or by other vendors.