The Theory
Camber is the angle between the wheel's centerline (seen from the front of the car) and a vertical line. It is measured using a camber guage and can be positive or negative. Negative camber is when the wheels lean inwards towards the chassis, positive is when the wheels lean outwards.

Negative camber is used to combat the effect of chassis roll. Chassis roll is where a car enters a corner, the inside suspension lifts and the outside suspension compresses, causing the wheels to lean outwards (positive camber). This is a problem because now only the outer edge of the tyre is touching the track, causing a loss of grip. Putting on a few degrees of negative camber means that when the car leans into a corner there is a greater surface area of tyre in contact with the track giving more grip.
So, what's the catch? Well, although 2-3 degress of negative camber all-round will give you good grip and stability in the corners, it won't give the same effect at other times. To achieve maximum grip when the car is going in a straight line i.e. when accelerating or braking, you need zero degrees of camber.
So, like all other elements of chassis tuning, it's a question of arriving at the best compromise.
NOTE: Positive camber is almost never used, so it won't be discussed in this article.

The Effects of Altering Camber
FRONT
More negative camber	Less negative camber
Lower centre of gravity	Increased ride height
Quicker steering response	Increased chassis clearance
More side traction	Less side traction
Reduced chances of traction rolling	Increased chance of traction rolling
REAR
More negative camber	Less negative camber
More side traction	Less side traction
More traction under braking	Less traction under braking
Reduced chance of traction rolling	Increased chance of traction rolling
	May give more steering