Punctuation is using marks in writing to make it easier to understand.<--That period or full stop is a punctuation mark.
Parts and Fancy Names
Don't worry if you don't understand these names~ they are hard to understand, but you'll get the hang of it!
. Is a period or full stop. Its fancy name in a sentence (English) makes the sentence a declarative or imperative.
! Is an exclamation mark. Its fancy name in a sentence makes it an exclamatory sentence.
? Is a question mark. Its name in sentence makes it an interrogative sentence.