If you are wondering what spotting is, it is the orange marker that appears on top of the enemies (infantry and ground/air vehicles)
How to do it? the default key is "Q" you press it when you aim at the enemy and poof...he is lit up like a Christmas tree
(not for long though.)The use of it?
- First, if you don't kill the enemy but your team does, you get points
(even if you are dead or very far away from the target)- Second, all your team sees the marker and knows where the enemy is at and where to pay attention to. (actually this is more important.)
example: if you are using recon and *cough*camping*cough* from very far away
make your priority to spot the enemies because maybe your team mates that are attacking at close range can't see the enemy squad that is about to flank them, and you can't be sure that you can wipe this squad before it reaches your team mates. 
So first do the spotting then you can go all "Vasily Zaytsev" on them
http://www.youtube.com/watch?v=wMvTR012DmgAnd you can apply this example to enemy tanks / helicopters etc. and also with any class, specially in close quarter fights, because sometimes engineers come with silenced weapons so their gun fire doesn't spot them on the mini-map, so spot them before you get killed to give chance to your team to find the person who killed you and knife him
maybe tea-bag him afterwards KIDDING! So...its a win win situation and you definitely help the other squads in your team to make strategies based on the position of the enemy and this actually makes the difference between winning or losing...
I hope this helped atleast a little bit, and If some of the more experienced players want to add something, please do, we are all here to help
