Well, they're both groups of tiny words which you really need to use.
Articles (a, an, the) mark nouns. They are part of a larger group called determiners (which include possessives, eg. my, her, their and so on)
Prepositions relate things to the subject. You need them when you're saying when or where something is (in, on, at, and so on), as well as adding an object when you have an intransitive verb.