Hammer on is the rock and roll term for an ascending slur.
You dont sound the note with your pick, the sound is created by rapidly, 'hammering on' with your fretting.
Same with pull offs, or descending slurs if ya wanna nitpick,
they are sounded by pulling down off the fretboard, sounding a lower note. Doing both these technique rapidly in succession is called trilling.
Anyway, why did you ask that question on this thread?