In guitar playing, the term "hammer-on" refers to putting a finger on a higher fret on a string that is currently being played on a lower fret. For example, while playing the 6th string on the 3rd fret, you can put your finger on the same string but on the 5th fret as if you're to "hammer" the string (Fig. 1); hence, the term "hammer-on".
Here's how a guitar hammer-on looks in actual guitar-playing:
On the other hand, the term "pull-off" refers to removing a finger positioned on a string on a certain fret; hence, to "pull-off". For example, while playing the 6th string on the 5th fret with your ring finger, remove your ringer from its position so that your index finger will assume the position of the string being played. See Fig. 2:
Here's how a guitar pull-off looks in actual guitar-playing:
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