Shareware programs are often restricted in one of 3 ways:
1. The program is time limited. It's a full version, but you only get 30 days (or so) to try the program before it won't run on your computer anymore. Of course, buying a license gives the serial number to unlock the program even if it's trial period is up.
2. Some programs fall into the category of "nagware". In this case, you get a full featured version of the program, but you are subject to a window that pops up and reminds you to buy the program - sometimes with annoying frequency. One version of this has an increasingly long startup time during which the "buy me now" message appears - the longer you use the program without purchasing it, the longer the delay at the beginning gets.
3. Many other programs "lock out" functions until you buy the complete version. Windowblinds, if I remember right, is one of these programs. You can use the shareware version as long as you want. But it will be limited in its features until you "unlock" it with a serial number.