Capture the Flag or CTF's are fun competitions and help bolster learning the following; Computer Cyber Security, Computer Networking, Cryptography, Stenography, Software and Hardware related challenges.
Teams of competitors (or just individuals) are pitted against each other in a test of computer security skill.
There are two kinds of CTF competitions. Jeopardy CTFs and Attack & Defense CTFs.
Here is a CTF Field Guide. Trail of Bits
What is a CTF? CTFd