Valve Software is the Company that designed Half-Life, and helped with Counter Strike.

Gearbox Designed the Opposing Force and Blue Shift expansion packs, and also helped with designing Counter Strike.

Counter-Strike is a multiplayer only Half-Life modification that pits terrorists versus counter-terror forces in a variety of locations.  The objectives are either for the counter-terrorists to rescue hostages being kept by the terrorists, the terrorists to bomb a particular target, or for the counter-terrorists to escort a VIP to safety.

Day of Defeat is a multiplayer only mod for Half-Life, set in World War II.  Players can choose either American or Nazi soldier classes, including Unteroffizer, Rifleman, Grenadier, Sergeant, etc.  Maps are either capture the flag, capture/guard secret data, or bomb/guard a target.  Settings include Normandy, Anzio, and other authentic European WW II battlefields.