Socks are just that, knee length socks. They can be any colour you want but most people choose white. Proper fencing socks are available with some padding on the front of the shin but are not required and this padding is not very effective, if you fence epee, you may want to think about getting a shin guard. Knee length socks are only needed when using breeches.
Football socks provide a good cost effective alternative to fencing socks at around half the price.