Donuteater2 said:
Title says it all really.I've heard the term used (mostly to complain about COD) but I don't know what they are at all.
Also, if they are bad, why are they used and what alternatives are available?
You've had it explained to you and you should easily find out for yourself.
Quite simply when programing how a gun works in a game, no projectile is calculated they take a short cut, the screen perspective just "scans" for where the reticule is pointing, and if it is pointing at the enemy's then for the next frame after the fire-button is pressed in the game it just changes it to "bullet hit there" and might also draw a line between the gun and where the target got hit like if the bullet had a tracer round.
The thing is this is so fast, faster than REAL BULLETS that travel at about 250-1000 metres/second. That may seem fast but that is as little as 16 feet per frame for pistol and shotgun projectiles in a game with 60-frames-per-second. That is enough that someone a reasonable distance away you HAVE to lead your shots.
Games like Team Fortress 2 have only a few hitscan weapons and they are limited by rate of fire, like the slow sniper rifle. Most weapons are "projectile based" as this means moving targets you have to lead your shots and your opponent still has a small chance to avoid being hit.
Hitscan isn't a huge problem at close range, but at long range since if someone if further away even though they are a smaller target you move through a smaller field of their arc as they jump and run and dodge around, this makes them much easier to track with a mouse and if the bullets always arrive instantly you just have to click on them and they don't stand a chance.