Full stops (also called »periods«) are not only used at the end of a sentence, but also in abbreviations and decimals.
Full Stop at the End of Sentences
Use a full stop to end affirmative sentences.
Example: I go shopping every day.
Use a full stop to end orders.
Example: Leave me alone.
Note: Americans often type two blanks after the punctuation mark at the end of a sentence.
see also: → Punctuation Marks that End Sentences
Full Stop after Abbreviations
The full stop after abbreviations is optional in American English, but not usually used in British English.
BE: Mrs Robinson, 5 kg, NBC
AE also: Mrs. Robinson, 5 kg., C.N.N.
Don't use a full stop after acronyms.
Example: NATO, AIDS
see also: → Business Letters (Salutation)