Music for the classic guitar is written out note for note, just like any other instrument. The range of the guitar (full range, three octaves and a fifth, from the E on the open 6th string to the B on the 18th fret on the first string). To avoid having to read two clefs like music written for the piano, guitar music is written on the treble clef only. To do this, the guitar is a "transposition" instrument in that the actual sounds of the notes are one octave below the note as written. For example, the C on the 5th string, 3rd fret is written on the first ledger line below the staff—middle C on the piano—but it is actually an octave below middle C. To play a middle C in relation to the piano, you find it on the 2nd string, 1st fret (along with several other places). Sounds more complicated than it really is. A lot of standard orchestral instruments are "transposition," i.e., the notes they actually play are not exactly as written.
The problem with keeping it all on one staff is that one has to learn to read notes on ledger lines above and below the staff. Sight-reading for the guitar is not too easy, because often the same note can be found several places on the fingerboard. For example, the aforementioned C on the 2nd string 1st fret: it can also be found on the 3rd string 5th fret, the 4th string 10th fret, or even up over the body of the guitar on the 5th string 15th fret. Where you would play that C depends on what else is going on. So unless a piece is pretty easy and sticks close to the first position, classic guitarists usually have to do a couple of read-throughs at least before being able to play through it smoothly.
But all the actual notes are right there on the page.