END PUNCTUATION
Periods.
Periods most commonly mark the end of a declarative or imperative sentence. They also follow at the end of indirect questions (Dirty Harry asked the punk if he was feeling lucky.) Periods are also used at the end of an abbreviated word to show that it is abbreviated (Dr., Mrs., Blvd., etc). Periods also follow letters in acronyms when those letters are pronounced separately (A.A.R.P., W.B.), but not when the acronym is said as its own word (NASA, PETA).
Question marks? And exclamation points!
Question marks indicate the end of a direct question (How far is the town?). Exclamation points indicate the end of an exclamatory interjection (Ow!) or sentence (He killed her in my bed!). In formal writing, these marks are always singular (so stop writing "It was a blast!!!"). If a direct question is exclamatory, use an interroburst, which is an exclamation point superimposed over a question mark.
Ellipses . . .
The final way to end a sentence is to use ellipses, or a series of three spaced periods. This indicates that a thought or sentence is trailing off (I couldn't quite remember . . . .) or when words are omitted in a quotation ("We all survived . . . survived despite the cost.") Remember that an ellipsis consists of three periods except when ending a sentence, where there are three periods for the ellipsis and one more to end the sentence.