Obviously, .com is based for companies, and its the most popular domain extension, so it became not only used by companies, it's used for several types of sites too.
.net is a good option for a network, hosting firm, commuications, any network related.
Well, some people will secure .net domain along with their .com for a simple reason, they want to protect their name much better so they register all common extensions at once.
Hope this is helpful a bit.
Ahmed