Sentences are typically classified by their purpose. What is this particular sentence trying to accomplish? In general, the purpose of a sentence is distinguished into three main types: interrogative, imperative and declarative. Interrogative sentences pose a question; for example, “Where is your dog?” Imperative sentences issue a command; for example, “Drop that spoon!” Declarative sentences present statements of fact; for example, “That is a dog.”
The most common variety of sentence in the English language is the declarative sentence. According to the Writing Centre at the University of Ottawa, “You can, and often will write entire essays or reports using only declarative sentences, and you should always use them far more often than any other type.” The declarative sentence is the most basic means for simply relating information or making statements of fact. One way of looking at the declarative sentence is that it's used to describe reality, to capture what is.
Generally, declarative sentences are simple in structure and easy to parse into constitutive parts. They are basically predicative statements that connect a noun to a verb or, put more simply, they explain the action of an object. They are also terminating, self-contained statements of fact that conclude decisively, with a period.
One way to understand a declarative sentence is that they might answer a question but they don’t naturally raise them; they declare, postulate, stipulate and explain rather than inquire or instruct. For example, “Your car is an older model.” Or, “The brown house needs a new roof.” In both cases some object is introduced and then subject to further elaboration.