Pleasure Pairs is when everyone teams up with another horse/rider combo. Usually you try to find someone with a horse that looks like yours, similar height, color etc. The class is done like a pleasure class except you are to ride beside your pair the whole time.
Pleasure Command ? uhm, i've heard of just Command? so im going to assume they're the same. Command is like a pleasure class except for when they say Canter, you are to canter right then, and unlike a pleasure class they can call for stops, back up, counter canters etc. People will get eliminated & ask to stand in the middle of the ring if they don't do what was asked right away. The class will keep going till there is only one person/horse left, and they are the winner.
Hope those make sense!