Punctuation ends sentences. Punctuation connects sentences. Punctuation separates a series within a sentence. Punctuation separates modifiers in a sentence. Punctuation encloses or emphasizes interrupting material.
Punctuation designates quotations from the rest of a sentence.
A semicolon joins two sentences when the idea in the second sentence is a continuation of the one in the first. A colon can connect two sentences when the second sentence contains an illustration of the first. I am legally an adult, but I am prohibited from drinking alcohol. A comma can also be used to separate words or phrases in a series. New York, Connecticut, and Rhode Island have raised the drinking age from 18 to 21 years old.