I don't really know what you mean by 'difference' but..
.com - Commercial .. used mostly for business type sites. The 'ultimate' in domain names, most highly prized generally speaking.
.net - A step down from .com as far as value goes. People who are not 'net savvy' might not remember this as well. Mostly used for 'network' related companies such as phone companies, ISP's, webhosts etc.
.org - Most .org domains are used for non profit organizations. Some commercial sites use .org's if the .com and .net's are not available.
.ca - I'm pretty sure this is Canada's TLD. Used for all purposes but can only be attained by Canadian residents.. I think.
There's also .edu for schools and other educational institutes, .co.uk for the United Kingdom, .au for Australia, etc. I think each and every country has thier own TLD. There's also the new .info and .biz . They're just extensions originally meant to tell the surfer what kind of site they were on, but are now kind of all blurred together.