Semantic features
The preposition is traditionally defined as a word expressing relations between words in the sentence. The weakness of the traditional definition is that it does not allow us to distinguish prepositions from subordinating conjunctions.
A new approach to prepositions and subordinating conjunctions is to treat the two traditional categories as prepositions (Geoffrey K. Pullum and Rodney Huddleston, 2002: 600). The said scholars include in the preposition category all of the subordinating conjunctions of traditional grammar with the exception of whether and that.
Functionally, prepositions can be divided into grammatical, and non-grammatical (the latter are subdivided into spatial and non-spatial).
Morphological features
Structurally, prepositions fall into two categories: simple, or one-word, prepositions (in, on, for, to, about, after, etc.) and composite, or two- or threeword, prepositions (ahead of, because of, according to; by means of, at the cost of, with reference to, etc.).
Syntactic features
As far as phrases are concerned, the function of prepositions is to connect words with each other. On the sentence level: a preposition is never a part of a sentence by itself; it enters the part of sentence whose main centre is the following noun, or pronoun, or gerund. It won’t be correct to say that prepositions connect parts of a sentence. They do not do that, as they stand within a part of the sentence, not between two parts.