Maybe this breakdown will help you understand the difference.
Stack Overflow is a Q&A site for programmers. This is where you can ask and answer questions about programming.
Stack Overflow became so popular that users wanted to create Stack Overflow-like sites about other subjects. Stack Exchange is the name of the network of sites which users have created. You can see a list of the sites created here.
Area 51 is the place where users can go to propose new ideas for Stack Exchange sites. Users post "sample questions" as part of the proposal process, and there is Discussion Zone for more extensive deliberations and discussions regarding the proposal as a whole. You can read more about the Area 51 process here. And here is a list of the proposed sites.