Largest structural unit in terms of grammar organization. There are many ways to classify sentences. There is using formal terms (declaratives, interrogatives, imperatives, and exclamative), functional (statement, question, command), and simple vs. complex and compound (terms of the number and kind of subjects).
Crystal, David. A Dictionary of Linguistics and Phonetics. The language library. Oxford, UK: Blackwell, 1997.
Matthews, P. H. The Concise Oxford Dictionary of Linguistics. Oxford: Oxford University Press, 1997.