Simple Sentence a simple sentence consists of just one independent clause: Mary had a little lamb. Compound Sentence

92. ABBREVIATED WORDS
Abbreviation is the most widely used term for a shortened or contracted form of a word or phrase. Abbreviations of single words are typically formed using the first letter or letters of the word ( n. = noun; adj. = adjective ), the first letter and the last letter ( hr. = hour; Sr. = Senior ), or the most significant letters ( TNT = trinitrotoluene; Pvt. = Private ).
Abbreviations may be nearly as old as writing itself; they allow a writer to save time, space, and effort. The cost of materials like parchment, paper, and ink was another major impetus to shorten words and phrases. Even with the invention of the printing press, cost remained important, and printers looked for ways to save space without diluting the message. Many abbreviations have become standard, including abbreviations for days of the week ( Mon., Tues. ) and months of the year ( Jan., Feb. ); common Latin terms ( lb., e.g. ); units of time and measurement ( min., ft. ); titles of individuals ( Mrs., Rev. ); and titles or names of organizations ( NCAA, UNESCO ), government bodies ( SCOTUS, EPA ), and states and cities ( Pa., NYC ).
The usual practice in American English is to use a period to end any abbreviation that stands for a single word (for example, assoc. or assn. for association ), whereas in British English the period is typically omitted if the abbreviation includes the last letter of the word. For example, in British writing the word association might be abbreviated as either assoc. or assn (without the period); likewise, Fr. is an abbreviation for France, while Fr (no period) is the abbreviation for Father (as the title for a priest).
Phrases are typically abbreviated by using the first letters or initial portions of each word or each important word, usually without any periods. Similarly, a single long word is sometimes abbreviated with the initial letters of component parts of the word. Unlike ordinary abbreviations for single words, which are almost always read as if the word were spelled out (as by reading “Dr.” as “Doctor” and “lb.” as “pound”), abbreviations consisting of initials are usually read as written—either letter by letter or as a single word. An abbreviation that is pronounced letter by letter, like FBI for Federal Bureau of Investigation or DOD for Department of Defense or TV for television, is referred to as an initialism.
Many abbreviations for phrases, however, are pronounced as words: for example, NATO for N(orth) A(tlantic) T(reaty) O(rganization) or radar for ra(dio) d(etecting) a(nd) r(anging). This type of abbreviation is called an acronym. Some acronyms, like radar, laser, scuba, and Gestapo, have become so accepted as normal words that most people are unaware of their acronymic origins. In many cases an official name may be chosen purely to create an appropriate and catchy acronym, as in the federal "Commercial Advertisement Loudness Mitigation Act" of 2010 (the "CALM Act").
In a loose sense, initialism can refer to any abbreviation composed of initials, even if pronounced as a word; conversely, acronym has been widely adopted to refer to any such abbreviation, even if it is pronounced letter by letter. But the distinction between true acronyms (pronounced as words) and pure initialisms (said letter by letter), is a useful one. To complicate the issue, however, there are hybrid forms—part initialism, part acronym—like CD-ROM ( [see-dee-rom] ) and JPEG [jey-peg] )—for which one term is as good as the other.
With the increasing popularity of email, text messaging, and social media, people—especially young people—have found new ways to save time and space, bond with friends through use of in-group jargon, and keep their communications opaque to prying parental eyes, by using initialisms to represent common expressions. Among the most popular are OMG (Oh my God), BTW (by the way), AFAIK (as far as I know), LOL (laughing out loud), ROTFL (rolling on the floor laughing), IMHO (in my humble opinion), FWIW (for what it’s worth), TTYL (talk to you later), and bff (best friends forever). BTW, IMHO, the rest of the population is catching on fast. OMG!