With hammer-ons and pull-offs you might find things like these :
E----------------------------------------------------------------
B----------------------------------------------------------------
G----------------------------------------------------------------
D----------------------------------------------------------------
A---------5h7-----------5h7--------------------------------------
E---0--0----------0--0-------------------------------------------
which would mean play the open E twice, then hit the A string at the
5th fret and hammer on to the 7th fret.
Pull offs look very similar :
E----3p0------------------------------------------------------------
B---------3p0-------------------------------------------------------
G--------------2p0--------------------------------------------------
D-------------------2---------------------------------------------
A----------------------------------------------------------------
E----------------------------------------------------------------
Here we have a descending blues scale using pull-offs to the open
strings. For each pull off you only pick the first note of the pair
with the right hand - so in this example you would pick all the
notes on the 3rd and 2nd frets, and the open strings would be
sounded by pulling off.
Because you give the string an extra bit of energy when you hammer on
and pull off, you only need to hit the first note with the picking hand.
You could even have a long string of hammer-ons and pull-offs like
this :
E----------------------------------------------------------------
B----------------------------------------------------------------
G---2h4p2h4p2h4p2h4p2h4p2----------------------------------------
D----------------------------------------------------------------
A----------------------------------------------------------------
E----------------------------------------------------------------
In this case you only pick the first note.