Then again it might not. It could nag you at random times when you're using the program or just try to make you feel very guilty every time the program starts up. Freeware is much like shareware, except you don't have to pay for it. Sometimes the motives aren't that altruistic, and a free software product is a dumbed-down version of a shareware or commercial product. What separates the first three categories from Open Source software is you don't get the program's source code when you download the program.
Source code is the human-understandable text that makes up a software product.
Often it can extend to literally millions of lines, which are fed through another computer program called a compiler. A compiler takes the source code and converts it to a file that a computer can run. As an average user, you probably wouldn't. As a programer it means you can actually see how a program works. You can fix bugs or extend the program's functionality by adding your own code.