There is more to a sock than you think. You can get sport specific socks for a wide range of sports now. Due to your feet being one of the main contact points on your bike they can be considered as important as a pair of gloves for many.
A summer cycle sock is usually thinner than a normal sock. They are made from high wicking fabrics to help wick sweat away and keep your feet dryer. There are some brands that take it a step further by adding compression bands around the arch of your foot and thicker pressure knobbles at the ball of the foot. These help prevent over stretching and reduce that pins and needles feeling from the vibrations of the bike. Some even have larger pressure knobbles that help to keep your toes spread apart to aid blood flow therefore reducing numbness.
For the colder months you can also buy waterproof socks. These are most popular for mountain bikers to wear to keep their feet dry in winter but brands such as SealSkinz and Dexshell do offer road cycling specific waterproof socks that are growing in popularity. Cyclo Cross riders also chose to wear waterproof socks in winter for muddy courses.
You can also buy team socks and this is an inexpensive way to show your support for your favourite cycle team and brands like Sock Guy offer some weird and wacky designs for those that just want that little something different!