[ehsankia]

I used to participate to a few as a hobby back in my university days. The online ones are generally jeopardy-styled, where you get a bunch of challenges across a variety of categories. Web, Crypto, Stego, Binary Analysis, Reverse Engineering, network packet analysis, etc. There's a bit of everything really, and they also try to have a range of difficulty with an appropriate point system.

CTFTime [0] is a great central hub for finding, tracking and reading about CTFs. After each CTF event, many people publish "writeups" which is basically their "walkthrough" of how they solved each challenge. You can try looking at those to get a rough idea of the various skill sets required for these events. The ctfs github group [1] is filled with them.

[0] https://ctftime.org/

[1] https://github.com/ctfs