Malware
Wait wait wait, why are bad things here?
Well, because you need to know about them and knowledge is the best defense. I have lumped in some things that are not necessarily bad/malicious, but that can often be exploited or used for nefarious purposes. You need to be aware of them so that you can use them to keep yourself safe, as well as understand the tools/techniques that are out there and possibly being used by malicious actors.
I will repeat: Do not use the following knowledge for illegal or malicious purposes. Dont be that guy.
Malware - For research purposes ONLY

Guides and Resources

Malware Sample Repositories

Botnets

Keylogger