A starting point, to introduce oneself to the main CTF categories.
Hacking websites.
The art of exploiting a computer program on the lower level.
Cryptography! Breaking old and modern ciphers.
Understanding how programs work by analyzing their code and behavior.
Investigating and understanding what has happened on a computer system, preferably before the hackers have escaped.
Finding and exploiting the bugs that were never meant to be found.
Challenges that don't fit in the other categories.