The document discusses various punctuation marks and errors related to punctuation. It explains that commas are used to connect elements within a sentence while periods end sentences. Two common errors are comma splices, which incorrectly join two independent clauses with just a comma, and fused sentences, which join two independent clauses without any punctuation. The document provides examples of these errors and how to correct them by adding connecting words or separating the clauses into separate sentences. It also discusses sentence fragments, which are incomplete sentences, and provides examples and corrections.