Question again...

whats the difference between using a decompiler(dissasembler) and reverse engineering

Reverse Engineering is working things backwards. (Guess)

Decompiler is where you can take the whole thing apart, pick what you want to mod, then assemble it back together (debugging)

(Obtw I just bs'ed that whole explanation XD)

Using a disassembler is part of the process of reverse engineering =)

A decompiler usually reverses intermediary code or bytecode found in Java and C# or other languages that run on a virtual machine. I believe it is also called a reflector.

A disassembler is a takes assembly code, the finished (.exe/.dll/etc.) that you might compile in C/C++ and reverses it back to the source for that code. Usually the code makes nearly zero sense.

So it takes a skilled patron in the art of reversing to figure out what is what. This is why you never hear of another software company reversing another's design. Usually the effort it takes isn't worth the reward.