Paragraphs are the building blocks of papers. Many students define paragraphs in terms of length: a paragraph is a group of at least five sentences, a paragraph is half a page long, etc. In reality, though, the unity and coherence of ideas among sentences is what constitutes a paragraph.
A paragraph is a distinct section of writing covering one topic. A paragraph will usually contain more than one sentence. A typical paragraph will be 5-7 sentences, but this is by no means a rule. The length is determined by the topic and the content.