Benji Acol (sometimes called Benjaminised Acol) is more of a system change than a convention, but its a pretty minor change. Benji Acol changes the meaning of the two level opening bids:

2	A strong two in any suit or 19-20 balanced
2	Any 23+ hand
2NT	21-22 balanced

So the main difference is that the super strong opening has moved from 2 to 2. That's because the 2 is now used to show any strong two hand, or 19-20 balanced. After you open 2 partner bids 2 and you then bid the suit of your strong two or bid 2NT with the balanced 19-20 hand.

Because of this extra option in the 2 opening the 2NT opening now shows 21-22 points. Opening 2/2 are weak two bids as before.

A strong two hand is one with perhaps 16-19 points and a very good suit. Another definition is eight playing tricks in a suit. Some examples:

A K J 10 8 7 3 K 7 4 3 A 5	Strong two in spades. Open 2 then rebid 2
K Q 8 7 A J 8 K J 5 A J 9	19-20 balanced. Open 2 then rebid 2NT
K 5 9 K Q J 10 8 A K Q 9 3	18 HCP unbalanced, open 1

Very similar to Benji Acol is Reverse Benji. That simply swaps round the 2 and 2 bids. I used to play that for a while, didn't really like it.