Sometimes developing a game can be challenging, especially when it comes to new ideas, which is why most developers have the "third person perspective with automatic wall cover" mode set to Alt+F7.
We're happy you asked. A third person shooter is a genre that applies to videogames. The genre usually involves action of some sort, where the players view is positioned behind the playable character or "avatar". This means it is much easier to empty your .45 into the face of your enemy from behind cover than it is in a first person shooter. It also means you have some sort of connection with the character; rather than the character being a mute, emotionless jerk, he/she is an identifiable, charistmatic jerk.
Hey everyone, JERK FIGHT!
Of course, a bland, emotionless character makes no difference if the gameplay is good. Unfortunately, developers all seem to share the same software when it comes to designing their game. Recently, the cover system has come into play, and, as usual in the videogame world, if one game has it, every game has it. Except, not every game has managed to pull it off correctly. Usually, instead of snapping to cover and ducking in order to avoid a bullet sandwich, the player shouts at the screen, frustrated that the character has slapped himself/herself against a wall which is in the direct firing line of the _________________ (insert generic enemy here).
If we're being honest, we're pretty sure everyone's copying Gears Of War, but we really don't care.
There may be some argument here as to what defines a third person shooter. Tomb Raider can be seen as a TPS, as it's in the third person perspective and you shoot things. Some platforming games, such as Uncharted, can be classed as TPS', since the view switches to the good old "over the shoulder" view when the player draws a weapon.
Resident Evil 4 is considered to have redefined the genre itself, thanks to the mix of shooting people in the face and jumpy bits that make even the most manly of players squeal in terror. As stated above, the "over the shoulder" shooting system was really brought into play with RE4, and has been applied to other games, such as Batman: Arkham Asylum (there's no shooting, but you can throw bat-wtf's at people).
Gears of War introduced the cover system, as we've already said, but no-one cares, since most cover systems don't work, and when pushed, most players will rush headlong at the enemy, shouting lines from Die Hard, whilst firing wildly into the air.
YIPPEE KAYEE, METAL-GEAR FUCKER!