High risk threats typically are remotely exploitable vulnerabilities, which can lead to system compromise. Successful exploitation does not normally require any interaction. May open up communication ports, use polymorphic tactics, stealth installations, and/or anti-spy counter measures. May use a security flaw in the operating system to gain access to your computer.
Misc - Anything (other than a document) not in another category, perhaps because it falls into mulitple categories, such as a tool suite.