The most common symbols used for slides are / for a slide
up and \ for a slide down.
You might also see 's' used to mean slide.
You don't always need separate symbols for 'up' and 'down' slides
since a line of TAB reading :
E----------------------------------------------------------------
B------7/9-------------------------------------------------------
G----------------------------------------------------------------
D----------------------------------------------------------------
A----------------------------------------------------------------
E----------------------------------------------------------------
is clearly a slide *up* from 7th to 9th fret. However you might
also see things like these :
E----------------------------------------------------------------
B------/7-9-7\---------------------------------------------------
G----------------------------------------------------------------
D----------------------------------------------------------------
A----------------------------------------------------------------
E----------------------------------------------------------------
where the exact start or finish of a slide is not given. Here you
have to know whether you're sliding up or down. In these cases use
your judgement to choose the starting or finishing fret. The effect
usually desired is to have a note 'swooping in' from a lower pitch
or dropping suddenly in pitch as the note fades.
You could have a whole series of slides running together, like this
E----------------------------------------------------------------
B------7/9/11\9\7\6\7--------------------------------------------
G----------------------------------------------------------------
D----------------------------------------------------------------
A----------------------------------------------------------------
E----------------------------------------------------------------
which would mean you only strike the first note with the pick using
the sustain to produce the other notes.