Every single word in a sentence will belong to a word class, there may be times where a word's job differs and therefore it will belong to a different word class than other times you have used it. Prepositions actually hold a very important part in a sentence, but aren't as noticeable as other word classes such as verbs or nouns for example. Interestingly, I have found that I took more notice of prepositions and the role they play when learning other languages.
So, what is a preposition?
Prepositions provide vital information to a sentence, either telling you where or when something is in relation to something else.
Here are some common examples of prepositions:
Outside, after, on, under, before, behind, inside, since, within...
Out of all the word classes, prepositions seem to be the most simple as they do one job and are used quite easily and most of the time without even realising. However, in true word class style, of course there is more than one type of preposition.
There are prepositions of:
Movement/ Direction - Up, down, on, over
Place - Inside, outside, over
Time - at, on, before, after
And they can also be counted as:
Simple - prepositions consisting of one word
Compound - prepositions consisting more than one word
(in front of, in between etc)
You may come across other ways/ types of prepositions too. I have witnessed different ways of categorising this word class. It can seem overwhelming but with prepositions, what's important is the role it plays and information it provides to the sentence. While it's interesting to know about the different kinds of prepositions, it's not vital information. The important thing to consider is finding the right one to provide accurate information.