These two terms I think begginners will see thrown around a lot on the boards, but it is very difficult to arrive at a consistent definition for them.

IMO, they are frequently mixed up. Expecially, people talk about ways of increasing intensity when they are really talking about volume and density. But I realize other people may think differently on this. Trying to keep it simple, here are the definitions I've arrived at. If anyone has any thoughts on this, please speak up.

Volume = The amount of work performed in an exercise session. Traditionally calculated as the amount of weight lifted multiplied by the number of sets multiplied by the number of reps (i.e. total reps), time under tension and amount of rest between sets can also be considered as ways to increase or decrease volume.

Intensity = Normally expressed as the percentage of weight you are lifting relative to your one rep maximum. In other words, how much weight is on the bar in relation to your maximal ability.