Player A will see a small targeted person icon next to the Player B's name on the scoreboard and on the kills feed. If Player A dominates Player B, Player A will become Player B's Nemesis. In CS:GO, It is only enabled in Arsenal: Demolition, Arsenal: Arms Race and Deathmatch.
A Domination is a 'reward' received for one player to kill, or assist in killing, one particular enemy four times without the aforementioned enemy killing or assisting in a kill against the player in Counter-Strike: Source and Counter-Strike: Global Offensive.