The term 'mob' screwed me up for a long time too...
I thought that the common use of 'mob' was to indicate a group of individuals... so when somebody told me I should be able to handle 2-3 mobs, I was thinking 2-3 'groups' of monsters...
So, just by playing I've learned that 'mob' means a single monster... and that 'adds' are additional monsters, like minions or helpers... guards, whatever who get aggro'd if the leader is bothered...
I'm still learning a LOT as well... lol, long way to go. Learned a lot of short hand... like 'gl', 'omw', 'hf', and plenty more... you'll figure them out... i'm just on the tip o the iceberg.