A pad is a sound that has a very long release or a long attack (this is for swelling effects). They typically have a ton of reverb/delay FX on them, but you can create a pad without either one.
I'm not familiar with what exactly you think is a stab, but based off the name I could assume it has a very quick release and attack meaning the sound hits instantly and cuts out very fast/doesn't sustain itself for long periods of time like a pad does.
Long Attack - Rising/increasing effect...right...?
i.e. Build ups and Tension to the "DROP" right...?
Long Release/Tail - Descending/decreasing effect...right...?
i.e. Build downs in ending a bar, measure or hook/chorus right...?
Thanks.