Transcribers(people who write the tab out)have little differences somtimes,but these are the major symbols(good idea to know what the song sounds like too)...
Bends-the sign for a bend is usually an line/arrow curving like a slope from the note thats to be bended.Above the line/arrow is usually a fraction(like 1/2) or the word 'full'.These refer to pitch...so if it says 1/2 then you bend that note a semi-tone up(a semi-tone is the distance of one fret from the one your on).If it says 'full' the it a whole tone up(that would be the distance of two frets up).
Vibrato-the sign for vibrato will look something like this ~~~~~ and will appear above or beside the note or notes.Bigger zigg zaggs will most likley mean bigger vibrato.
Slide-the sign for a slide will be a slightly horizontal line from one note to the other and another arched line above those same notes.the sign SL. will appear above the arch.If there is no arched line,then it means you should hit the note your sliding up to.For online tabs there may just be an S between notes.
hammer on-The sign for hammer ons will be an arched line from one note to another and have an H above it.Online tabliture may just have an H between notes.
Pull off-the sign for a pull off is the same as a hammer on except there is a P above the arch.Online tab may just have a P between notes.
Palm muting-the sign for palm muting will appear as P.M. follown by periods to mark the notes it applys too(P.M. ............. for example)It will be places above the 6 lines or below.
I think that covers the basics.Hope it helped....cause if it didn't *shaking fist*
Try once,fail twice...