A conjunction is a word that joins two independent clauses, or sentences, together.
Example 1: Ellen wanted to take drive into the city, but the cost of gasoline was too high.
Example 2: Richard planned to study abroad in Japan, so he decided to learn the language.
In the examples above, both but and so are conjunctions. They join two complete sentences with the help of a comma. And, but, for, or, nor, so, and yet can all act as conjunctions.