"Killzone is a series of shooter games exclusively for Sony Computer Entertainment's video game consoles. The series is developed by Guerilla Games, a subsidiary of SCE. Killlzone consists of three games for the PlayStation 2, the PlayStation Portable, and the PlayStation 3 respectively, with a fourth currently in development for the PlayStation 3. The series began on the PlayStation 2 in November 2004 with Killzone, and continued on the PlayStation Portable in October 2006 with Killzone: Liberation. The most recent installment is Killzone 2, released in February 2009 for the PlayStation 3. Killzone 3 has recently been announced for a 2011 release also on the PlayStation 3.
The series is about the war between the Interplanetary Strategic Alliance (I.S.A.) and the Helghast. The protagonists of the series are Cpt/Col. Jan Templar (Killzone and Killzone: Liberation) and Sgt. Tomas "Sev" Sevchenko (Killzone 2 and Killzone 3) while the main antagonist is Helghast Emperor Scolar Visari, although his death in Killzone 2 brings about rise for a new antagonist in Killzone 3. The Killzone series follows the continuous war between the ISA and Helghast taking place on both I.S.A. Earth colonies and the planet Helgan which is the home planet of the Helghast.
Killzone, Killzone 2, and Killzone 3 are first person shooter games developed by Guerilla Games and published by Sony. Killzone: Liberation is presented as a third person shooter and is the only of that kind in the series to date.
Players in the series can only carry two different weapons at any given time. Players can either obtain ammo or swap out their current weapons with any weapon dropped by a downed foe or from those scattered around the various maps.
Online competitive multiplayer features up to 32 players in Killzone 2. There are various modes including assassination, deathmatch, etc. There's also a rotation mode where you play all game modes one after the other."
Source: Wikipedia, "Killzone_(series)", available under the CC-BY-SA License.