The various parts of a web address are separated by dots. The top-level domain is the last part of an address. In www.antiviruslab.com that would be ".com". The top-level domain represents the highest level of name resolution. TLDs that are valid internationally are: .com (worldwide, commercial), .mil (military), .gov (US and UK government), .org (non-commercial organizations), .edu (US and UK educational institutions), .net (net administration agencies), .int (international agencies), .info, .museum, .name (individuals), .coop (cooperatives), .aero (aviation organizations). Other top-level domains are only valid in specific countries: The identifier is a 2-letter combination. Here are the most common: .de (Germany), .at (Austria), .ch (Switzerland), .fr (France), .nl (Netherlands), .pl (Poland), .es (Spain), .ca (Canada).
The Top Level Domain refers to the extension such as .com, .net, .org, .gov, .edu, and so on. The second level is the unique name before the .com.