SSDT hook: redirect a syscall (e.g. NtQueryDirectoryFile) to hide artifacts.
Trojan: disguised as something benign; delivers a hidden payload. Example: fake "installer" or cracked app.
RAT (Remote Access Trojan): gives the attacker interactive remote control — keystrokes, screen, files, shell. Examples: njRAT, DarkComet, AsyncRAT.
Rootkit: hides its presence by subverting the OS — hooking APIs, filtering process/file listings, or living in the kernel. Example: Necurs kernel rootkit.
Rootkits are Lab 5 territory: they attack the very tools we use to observe them.