An interrogative sentence asks a question and hence ends with a question mark. In speech, it almost universally ends in a rising inflection. Its effort is to try to gather information that is presently unknown to the interrogator, or to seek validation for a preconceived notion held. Beyond seeking confirmation or contradiction, sometimes it is approval or permission that is sought as well, among other reasons one could have for posing a question. The one exception in which it isn't information that is needed, is when the question happens to be rhetorical. While an imperative is a call for action, an interrogative is a call for information.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/interrogative-sentence