I feel this is pretty important, especially for beginners at guitar. There's probably a lesson on it somewhere, but I thought I'd type out a full tab key. If I've missed out any, or explained them badly then say, but I just thought it might be helpful to people as I've seen a lot of questions asked about simple tab notations.
Strings - Unless stated on a tab that the song is in a different tuning, this is what the strings represent (standard tuning)
'E' being the fattest/deepest string, 'e' being the thinnest/highest. The strings are also called 'staff's when written out in tab form.
3h5 - 'h' is the symbol for a hammer-on. In the examples case, you would strike fret 3, then with another finger play fret 5 having only picked the string once.
5p3 - 'p' stands for a pull-off. Basically the opposite of a hammer-on, where you would play fret 5, then pull off your finger to create the tone of fret 3 having only picked the string once.
3/5 or 5\3 - The slash symbols mean (slide) that you play the first note, then with the same finger, move across the fret board to the next fret position having only picked the string once.
x - an 'x' on any of the strings means mute. This is created by lightly resting your finger(s) on the string, it shouldn't produce the sound of any note.
~ - Let ring. Basically hold the note/chord to let the sound continue on.
4^ - bend. Usually 1/4, 1/2 or full tone bends, meaning you bend the string (example) at fret 4, up or down, whichever you find easier.
A 1/4 tone at fret 4 should produce a sound of between frets 4 & 5.
A 1/2 tone at fret 4 should produce a sound of fret 5.
A full tone at fret 4 should produce a sound of fret 6.
^4 - pre-bend. Basically have the string already bent up and then resolve it so that the string is not now bent.
b - also meaning bend
r - meaning resolve bend, you will most likely only ever see this symbol alongside the 'b' symbol.
Harmonics - most common ways of playing them are either... (Natural) Lightly rest your fingers above the frets (works best at frets 5, 7, 12), pluck, and it should produce a sound that will stay even without your finger on the string.
The other way a harmonic (pinch) is played (more often like this in solos), put your finger on the fret board at any position, then pick at the string around the pick-up closest to the neck (works better) and lightly let your thumb catch the string as well. You will know when you've got it right, takes a bit of practice though.
(t)12p7p4 - The 't' symbol is usually shown underneath the tab staff lines, but it means tap. This is used in long pull-off stretches using your picking hand, literally tapping your finger onto the highest fret position. There is also two finger tapping which is more complicated and you should know normal tapping before attempting.
v - Vibrato. It's a fast amount of small bends on a string (note) to make the note resound for longer periods on time. On electric guitars bend rapidly up & down, on acoustic, left to right (you don't have to, but they're the better options for the types of guitar)
PM... - Palm muting, meaning you mute the strings around the pick-up area of your guitar with your picking hand. Quite common on punk-rock songs in chord verses.
Gliss - This is a sharp slide up, or down, the fret board.
Pick slide - basically scratching your pick along the strings, comes off best with a bit of distortion through your amp.
Coda - This is more often written in tab books to save space, however some tabs have it, and it means that you have to go back to a certain part of a song, basically an easy way to explain the order.
Trill - rapidly alternating between the notes indicated using hammer-ons and pull-offs.
Bar Dive - Use whammy bar on guitar to drop the note to a certain pitch.
Tremolo - Pick as fast as you can (up and down) on the notes indicated.
Rake - Dragging the pick across the strings noted, not quite as fast as playing a chord.
Arpeggio - Basically picking out notes from specific chords.