Counterstrike, originally created by Minh Le and Jess Cliffe, started in 1999 as an inofficial mod to Valve's successful 1st person shooter Half-Life and quickly became one of the most well-known standalone video games in the world and integral part of the e-sports community. Valve now holds copyright to the game and its current version 1.6 needs to be run on Valve's gaming platform Steam.
The game revolves around moderately paced tactical teamwork and does not feature a story. The setup is as follows: Two hostile teams, called Terrorists (Ts) and Counter-Terrorists (CTs), compete against each other in real time, but on a per round basis, meaning that once a player's character is dead, he will not be respawned until the current round is over by victory or respective loss of either one team, the victory conditions depending on the map type.
Common setups are:
The Ts have to set a bomb they spawn with at either of two predefined locations or enable the destruction of the sites by eliminating all of the enemy team. CTs have to prevent them from doing so by either killing all of the enemy team before the bomb is planted, holding them back until the timer expires or defusing the planted bomb.
CTs have to rescue hostages in time while Ts fight them.
Due to the low ressource requirements and fluent gameplay Counterstrike 1.6 is still being played and thus maintained by Valve. However, there have not been any changes to the existing version since 2003.