Learn English – How would one use the word preposition

"Simply put, a preposition indicates a relation between things mentioned in a sentence."

For example, in the above sentence, between is a preposition. There's the noun "relation", and the noun "things". The word "between" introduces a relation between them.

Wiktionary defines "preposition" as follows:

A closed class of non-inflecting words typically employed to connect a noun or a pronoun, in an adjectival or adverbial sense, with some other word: a particle used with a noun or pronoun (in English always in the objective case) to make a phrase limiting some other word.

Merriam-Webster says:

Definition of PREPOSITION: a function word that typically combines with a noun phrase to form a phrase which usually expresses a modification or predication

If you are still not quite sure what that all means, there's a list of English prepositions over at Wikipedia.

Also, as Kosmonaut points out in his comment, prepositions usually come before the words they modify — hence the name, "pre" + "position". However, note that there are mechanisms in place that allow prepositions to be "detached" from the words they modify; see, for example, these questions:

• How to combine in a sentence two verb + preposition pairs that have the same object?
• When is it okay to end a sentence in a preposition?

You can browse the full list of questions tagged "prepositions" for further insight.