Prepositions describe the relation between objects.
The book is on the table.
The book is under the table.
As you can see the preposition on/under describes the relation of the table and the book (this time in a view of space and place).
Prepositions can be quite tricky, because quite often there is no translation that works in all of the cases. Often you will see that the logic is different in different languages:
Il est faché contre moi. = He is upset with me.
but the French says: against me.
Elle se moque de moi. = She is mocking me.
but the French says: she is mocking of me...
And sometimes the French does not use a preposition, where English would need one:
Elle attend le bus. = She waits for the bus.
the French says: she waits the bus
There are about 150 preposition in French (as well as in English). There are some prepositions that are used in almost every sentence, as à or de. Then there are prepositions formed in only one word, as à, de, sur, devant etc. and others formed from more than one word, as à bord de, au bout de, à cause de etc.
We have tried to collect the most used ones and the most difficult ones. This does not means that the list is complete. However, it is a good start.