Tablature, Diagrams, and Copedents
Tab
Tablature is a simplified music notation system. Steel Guitar sheet music started using this system in the 1920's, though classical notation remained the dominant system used in steel guitar sheet music until the 1960's. Guitar and some other stringed instruments have used this system as well, at least since the 1960's. Here's what it looks like:
The lines represent the strings. The top one being string number 1 (the one furthest from the player). The numbers on the lines represent the fret where the tone bar rests when that string is plucked. The letters underneath indicate any pedals or levers used (pedal and lever names are discussed in more depth in the copedents section of this site).
As you can see, there is no indication in tablature on how to play the rhythm of the piece. Also, suppose a violinist wanted to play a piece written for steel guitar, how would they do so without learning how to read steel guitar tablature? The limitations are clear, but there are benefits to tab. It is really easy to read and understand, and it can be a really quick way to get the general idea of how to play simple tunes.
Diagrams
Sometimes diagrams are used to explain musical ideas. These are usually used to show scale or chord patterns. Diagrams show the fret board with markings on it indicating where the notes are. Diagrams usually don't show rhythm or music as it moves through time, but rather show where a set of notes would be located on the instrument.
Here are two diagrams showing some possible locations of the C major scale notes on the PSG neck:
The numbers over the top are the fret numbers. The spaces between the lines are the strings. The note names are written in the spaces at their proper frets.
Copedents
The steel guitarist's "copedent" is also a diagram conveying musical information. It is addressed in detail in the COPEDENT section of this site.