Abbreviation is the process of shortening something, usually a word or phrase, or the shortened version itself. “Mr.” is the abbreviation of “mister,” NASA is the abbreviation of National Aeronautics and Space Administration, etc. (hey, that's an abbreviation too).
There are different types of abbreviations. Some are followed by a period (.) to show that the word has been shortened. Acronyms, like DNA, SAT, or NATO, which are abbreviations that are formed from the first letters of a series of words, are written in all capitals. And don’t get confused: if your friend asks to give you an abbreviation of his speech, he’s not going to shorten each word; he’s just going to leave the details out.