Semantic features
Every conjunction has its own meaning, expressing some connection or other existing between phenomena in extralinguistic reality.
Syntactic features
Two levels are distinguished — that of phrases and that of sentences.
On the phrase level it must be said that conjunctions connect words and phrases. It is the so-called co-ordinating conjunctions that are found here, and only very rarely subordinating ones.
On the sentence level it must be said that conjunctions connect clauses (of different kinds). Here we find both so-called co-ordinating and so-called subordinating conjunctions.