.com is the commercial extension. Typically, .com domains are worth the most.
.net used to be a “network” extension; however, it fits forums well and is used when the .com version of a domain isn’t available.
.org is the most ‘restricited’ of the top 3 TLD’s. By ‘restricted’, I mean that many .org names aren’t worth much at all unless they are obviously useable for a nonprofit organization. Recently, however, with many other TLD’s being taken, the value of a good .org has increased.
.biz is the business extension. Although not widely used, some businesses do have .biz domains. A commercial keyword ending with .biz can be quite valuble.
.us means .United States. Domains that would be usable to a United States reseller aiming its products at the U.S. market can be worth a good amount in .us.
.info domains are an intereted batch. .info domains can be registered for under $3; additionally, a premium .info domain is typically worth less than a premium .com or .net. There are exceptions; some buyers do indeed value .info’s over other domains because of factors such as universality.