What can be contrasted with which, in that what refers to things that are unrestricted in number whereas which refers to things that are restricted in number (there may be a better technical explanation for this). There is some overlap however and in some contexts either "what" or "which" are acceptable.
What can be used as the head of a noun phrase with another noun phrase, e.g. "what time" or "what thing". It can also form questions without do support, with hidden do, e.g. "what happened?" (instead of "what did happen?"), or "What eats wasps?" instead of "What does eat wasps?" - but only when "what" is the subject. Which, who and whose also do this but how, when, where and why don't. There should be a technical explanation; the non-technical explanation is you can do this with people and things but not concepts (time, places, ways, reasons).
It forms the head of "whatever" and "whatsoever".
The final /t/ sound is often reduced to a glottal stop; /wɒʔ/
It is also an interjection; "What?!"