1. A preposition is a member of a word class which, as its name suggests, cannot stand alone. It occurs in "pre-position" to some other item which acts as prepositional complement (also called prepositional object) and completes the prepositional phrase.
Prepositional complements may be any of the following (preposition in bold, complement in italics, whole prepositional phrase underlined):
a) A Noun Phrase
We're going on Tuesday, probably in the afternoon.
Stop looking at me like that!
I found out by googling it.
c) A wh- clause
We were astonished by what we saw
We watched from above.
I can't do it by then.
In some constructions the prepositional complement may occur earlier in the sentence, but can always be retrieved. Eg :
That's the book I was looking for = I was looking for the book.
2. Prepositions may be simple - ie composed of one word (at, by, on, over, with, despite etc) or complex - ie composed of more than one word (in spite of, away from, because of). As these three examples show, complex prepositions generally fall into one of three categories :
a) Preposition + noun phrase + preposition : as a result of, in view of, in line with etc
b) Adverb + preposition : apart from, out of, as for etc
c) Verb or adjective or conjunction + preposition : owing to, regardless of, because of etc
3. Prepositions may also be correlative - ie composed of two parts. For example from/to, between/and
From An ELT Notebook...
An ELT Glossary : Multiword Verbs
Prepositional Verbs and Verbs with Prepositional Phrases. What's the Difference?
And without doubt the best grammar of English ever written :
Quirk et al, (1985) A Grammar of Contemporary English. Longman Chapter 6