Here's the scoop: You've got two different computer widgets mixed up here, and understandably so.
First, there are Internet domains, used to identify computers on the Net. You've got domains like yahoo.com and microsoft.com and whitehouse.gov, the part after the "dot" being (theoretically) indicative of the nature of the individual or organization that has registered the domain. Common "top-level" domains are .com for commercial, .govfor government, .org for non-profit organization, .net for network, and.edu for educational institutions.
There are also hundreds of country-specific domains (such as .ca for Canada, .au for Australia, etc.), plus the powers that be are planning on adding seven more generic top-level domains. So, in partial answer to your question, there are hundreds of top-level domains, although the most common four or five make up the majority. For more, check Yahoo!'s Domain Registration category (under Computers > Internet).
On the other hand, file extensions such as .html and .doc and .exerelate to individual computer files and are determined by software developers. For instance .html is used to identify a file as readable by a web browser. It stands for HyperText Markup Language, hence the H-T-M-L. Whereas, .exe stands for an "executable" file. Here again, you've got hundreds, perhaps thousands of different extensions, with the most common ones cropping up again and again. For a quick look at those recurring extensions, check out
