Rock and pop sheet music will range from simple guitar and vocals to full instrumental scores, and from popular single songs to complete albums. You might also see artist compilations, complete works and volumes focusing on specific time periods of artists, or compilations of a genre or era. If you're looking for a certain piece to play along to, there's a good chance it will be in one of the many books available.
Difficulty is an important factor when choosing sheet music. Although individual songs on sheet music tend to be pretty good interpretations of the original version, books can vary according to the audience they are aimed at. Beginners books make up a large proportion and are designed to be used as a learning tool, teaching your chosen instrument by playing songs you love rather than some of the songs that have been used for many years by music teachers but fail to inspire today. Some care does need to be taken though. Sometimes the songs are not meant to be played along with the recorded versions, but are to play solo or sing along to. Take the guitar, for example, songs in certain keys are generally much easier to play than the keys in which they were originally recorded, so songs written in these harder keys could be transposed to make learning simpler. Similarly, songs whose sheet music is in the same key as the recordings might use easier versions of some tricky or difficult chords, so the song can be played along with the recording, but might sound slightly different.
Also traditional music notation is not always used. For example guitar music typically has its own notation, using one of several variations. They are explained next.
Very common is the chord diagram style, which use a small picture representing the six strings of the guitar (vertical) and several frets (horizontal), with black dots showing how to place the fingers. With this notation you can just follow the diagrams without needing to know about notes or chords or even keys.
Second is tablature, usually called tab. This uses six continuous lines (four for bass guitar) just like typical sheet music, but instead of notes there are numbers to show the fret onto which the string should be pressed. The tab method lets you play both tunes and chords without the need to to be able to read music. The downside with tab and chord diagrams is that the music can only be played on the instrument it is written for.
With the third method, individual chords are named and written above the normal music whenever the chord changes. This has the advantage of allowing the chords to be played on any instrument, but it does create the requirement for the player to understand what each chord means. This chord method does not have the scope of describing intricate sections in full detail though.
Picking up music for popular songs is a great way to learn an instrument, and as you become more proficient on your instrument you can learn songs just as they were originally recorded. Understanding songs using sheet music can be a much faster way of learnig than trying to pick things up by ear.