Definition - As the name suggests, conjunctions are words that join other words, clauses, or sentences:
             • y [and], o [or], sino [but], si [if], porque [because], ...

Types - There are many types of conjunctions that indicate different kinds of connection: inclusion, alternative, exclusion, contrast, time, manner, cause, effect, purpose, condition, doubt, etc. Sometimes pairs of conjunctions are used together to connect different words or groups of words within the sentence.
Coordinating conjunctions link clauses of the same kind, for instance main clauses that express ideas with the same importance in the overall meaning of the sentence.
Subordinating conjunction introduce subordinate clauses, which are clauses that depend on the rest of the sentence to make complete sense.

Examples of Spanish conjunctions:
             • El tren parte de Salamanca y va hasta Madrid.
                [The train leaves from Salamanca and goes to Madrid.]
             • Este restaurante es bueno, pero es caro.
                [This restaurant is good, but it is expensive.]
             • ¿Son las tres o las cuatro?
                [Is it three or four o'clock?]
             • Ésta no es ni una maleta amarilla ni una bolsa roja, sino una cartera marrón.
                [This is neither a yellow suitcase nor a red bag, but a brown wallet.]