I've always been interested in the word preposition and at times, had used it correctly. How would one use it?
"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:
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:
You can browse the full list of questions tagged "prepositions" for further insight.