I have been wondering, if I get infected with a RAT, what can the hacker do to my computer? What will he/she have access to, and what can they do with my computer?
Taken from infosecinstitute.com:
when used for malicious purposes, they are known as a Remote Access Trojan (RAT). They can be used by a malicious user to control the system without the knowledge of the victim. Most of the popular RATs are capable of performing key logging, screen and camera capture, file access, code execution, registry management, password sniffing etc.
Since there are many varieties of RAT's, I suggest you start by reading the article I quoted and later if more information is needed a simple Google search will return a lot of information/research's that are relevant.