Internet, .com .net .org

Those are all "top-level domains".
.com, .net. .org, .mil (for US military), .us (for United States), .info, .biz, .name (3 newer ones that, IMHO, are all stupid!!!) and the country code domain (.uk for United Kingdom, .CA for Canada, .MX for Mexico, etc.)
THis is how the domain naming convention works. There are the top-level domains first. Under them, there are the 2nd-level domains (YAHOO under .com, RENT under .net, MOZILLA under .org, etc.) These are the domains that people can register under the top-level domains.

Go to www.internet.net and read the FAQ for more info.

hope this helps

.com = commercial web-site (for profit)
.org = orginization web-site (non-profit)
.edu = education (schools)
.net = network (often ISP's)
.gov = government

.com is open to everyone.
You need to provide proof of quailification for .org .edu or .gov

Chas