Google.com is a domain name, the extension ‘.com’ is known as Top Level Domain (TLD).
Some TLDs are opened and free to be registered by the publics, for instance: .com, .net, and .info; some TLDs, on the other hand, are governed strictly by government and cyber regulations, for example .gov can be only used by government organization and a country code TLD (ccTLD, like .co.jp and .co.uk) can be only used by websites located in that particular country. Some TLDs are intended for certain special purpose, for example .org is intended for organizations’ website (though it’s widely misused by personal websites).
Subdomain is the name in front of a domain name. For example ‘mail.google.com’, the word ‘mail’ is a subdomain. Subdomain is often confused with an addon domain. An addon domain is the second, third, fourth… independent domain name added in a single hosting account; a subdomain, on the other hand, is a secondary domain appears in front of a domain name.
Addon domain requires the owner to renew its registration; while creating subdomains require no cost at all.