Don't play the tab if you've never heard the song... Download or buy it first and listen carefully...
The letters represent the note you play "+" (or "#" ) are sharps, "-" (sometimes "b" ) are flats 1 2 and 3 represent octives Notes in Parentheses followed by * and a number represents how many times to play that phrase
<Added>here is a horn tab viewer
http://altrobe.net/stuff/tabview/
and to find horn tabs you can go to this web site
http://horntabs.net/
that is the only website i know for tabs