TLDs or Top Level Domain refers to the .com part of the address. When I started in internet marketing, there were only a few of these. There was .mil (military), .gov (government), .edu (education), .com (commercial), org (non-profit organization) and .net (network provider). This list has grown substantially, with .biz (business), being and early addition (Wikipedia Article).
Some countries took advantage of their catchy TLD (.me -Montenegro) and sold domains with their TLD generically. This has led many to believe that all TLDs are equal, but I assure you they are not!
Many TLDs are country/location specific and Google takes this into consideration when it’s delivering search results. Matt Cutts has actually spoken about this on several occasions, but this is the first time I have seen the corresponding list of what Google considers to be country specific and generic TLDs.
I should add, that the controlling board (ICANN/IANA) has dramatically expanded the number of TLDs and new catchier TLDs such as .travel, .info, .mobi are being added at a rapid pace!