From Wikipedia:

A country code top-level domain (ccTLD) is an Internet top-level domain generally used or reserved for a country (a sovereign state or a dependent territory).

All ccTLD identifiers are two letters long, and all two-letter top-level domains are ccTLDs.

Creation and delegation of ccTLDs is performed by the Internet Assigned Numbers Authority (IANA)

Some sample ccTLDs are: .de, .us, and .cn.

gTLDs are instead generic top-level domains. The more important are: .com, .info, .net, and .org (biz, name, and pro are restricted by some elegibility requirements).

The main difference is that ccTLDs contain a geographic designation.