Severe threats typically are remotely exploitable vulnerabilities, which can lead to system compromise. Successful exploitation does not normally require any interaction and exploits are in the wild. There exists a high possibility of potential system damage or security flaw. Attacker has complete control over your computer or install new software on your machine.
Misc - Anything (other than a document) not in another category, perhaps because it falls into mulitple categories, such as a tool suite.