Conjunctions are words that join sentences and clauses. There are three main types of conjunctions: coordinating conjunctions like "and", "but", and "or"; correlative conjunctions which always come in pairs like "not only...but also"; and subordinating conjunctions which introduce dependent clauses like "because" and "though". Conjunctions help vary sentence structure and smoothly connect ideas. It is important to use conjunctions appropriately - the meaning and flow of the sentence should not be contradicted by the conjunction used. Conjunctions are a useful tool for enhancing writing.