Translation:We have coats.
why "tener" becomes tengo, tienes, tiene, & tienen. not teno, tenes, tene & tenen?
It's irregular just like to have in English. "I have; he has." There has been some linguistic work done on why the most common verbs (to be, to have, to go) are often irregular. This wiki article mentions it: http://en.wikipedia.org/wiki/Regular_and_irregular_verbs
Upshot is we just have to memorize irregular verbs.
I thought it was "I have coats" , what is the difference between nosotros tenemos, and tenemos?
You would use this construction if someone claimed before that you do not have coats - a counter-statement - or to put some emphasis on the verb. Those uses also have a special form in Spanish: "Nosotros sí tenemos abrigos."