Just like challenge sites, CTFs are also computer security competitions. There are 2 types of CTFs: jeopardy and attack-defence. So far I've only played jeopardy ones, and because they are very similar to challenge sites, in this post I will mostly write about them :P
In jeopardy CTFs, you are given a number of challenges in different categories: crypto, stegano, web, pwn, RE, forensic... By solving these challenges you will get special passwords called "flags". Submitting these flags will earn your points. Unlike challenge sites, CTFs are events that usually last for 2 days, so it is recommended that everyone should play CTFs in teams. The teams with the most points will win the CTF. CTFs are a fun way to solve challenges, and many CTFs even offer cash prizes for the top teams.
Most CTFs have qualifier and final rounds. Usually the qualifier rounds are online, and final rounds are on-site. The biggest CTF is DEF CON CTF, which comes together with the famous conference that everyone already knows about.
My team is penthackon. It is a new team consisting mostly of high-ranked players in the WeChall network, and as a result we have done quite well and is currently #6 on the ranking. We even pre-qualified for DEF CON, and some of us will be going to Las Vegas in August to pwn there :P
