Like other sense-related verbs, it can be a copular verb.
In terms of meaning, "smell" can mean both "(give off a) stink", and "to sense a smell". Hence the sentence "I smell a smelly smell that smells smelly" parses. There may be a technical word for this in semantics, but I'm not sure what it is.
As a verb, it is an ambiregular verb. Smell has the third person form "smells" (/smelz/), and the -ing form smelling. It has the regular preterite and past participle "smelled" (/smeld/ or /smelt/), but also, particularly in British English has an irregular preterite and past participle "smelt" (/smelt/)
Of course, one of the many aspects of teaching teenagers, particularly teenage boys who have yet to discover deodorant, is that they can often smell quite bad.