Counter-Strike: Global Offensive is a 5v5 game made by Valve. The main modes in the game emphasize a 5v5 war between Counter-Terrorists and Terrorits. The CT's and T's have largely different strategies that can be used to win each side. After 15 rounds of a match, the sides will then swap and CT's will become T's and vice-versa.
I have put some good references in the navigation bar to read up on Counter-Strike: Global Offensive for those who do not know about it and would like to learn more.