bruno,
SAP has provided some standard functionalities. A company can use these for their day to day transactions, reporting etc. But in case company feels that there are gaps, the specific development can be done in SAP to meet these gaps. These specific developments will be applicable only for the company and not for all other companies using SAP. These are called SAP Enhancements.
The gaps are first documented (called functional specification document: FS), then the code is written to implement this FS. SAP has given some "places" where these codes can be written called USEREXITS
User Exits In Sales Document Processing - System Modifications - SAP Library
After coding, testing is done. If ok, the code (change request) is moved finally to production system.
Your thesis should include real time examples which showcase the benefit of SAP Enhancements. You can find examples in various forums (based on modules, SD, MM, FI) in SCN.
Enhancements involve cost - both in implementation and in support
I have seen several companies that do not have proper documentation of these Enhancements which causes problems, at a later stage. In some cases, Enhancements have never been used by business (there could be many reasons)
TW