There are two kinds of bike gloves, ones to keep your hands and fingers warm and the smaller fingerless mitts that keep your hands comfortable on the handlebars.

Whichever kind of gloves you use they have to be strong so that they protect your hands in case you fall off your bike. For the winter full gloves with fingers and made of a thermal material with strengthening patches on the palms of the hands are best.
For the summer months fingerless mitts with gel pads in the leather palms and Lycra on the back with maybe a towelling section to wipe the sweat from your brow so it doesn't get in your eyes. The backs used to be made of chamois leather or more often of crotched string; these were cheaper but kept your hands cool.
Handlebar tape can hurt your hands on a long ride; this is why most race cyclists wear them in big races like the Tour de France.
As like all other clothing, cycling mitts are another place to put team advertising, when Mario Cipollini or Simoni used to win a lot of races he would put both hands in the air as he crossed the line and you would see his sponsors name on the palms of his hands.
All the teams also put the sponsors name on the back on the mitt and you can buy the same ones to look the same as you're hero.
Why Wear Cycling Mitts?
Bob has asked us why do professional cyclist
wear those little mitts? There are a few reasons for this and all of
them are important. If you want to find out a little more about the
life of a Cycling Pro then read our explanation
Question: I've been watching professional cycling now for a few
years but I can't understand why they wear those little cycling mitts.
Thanks, Bob.
Well Bob, there are some very good reasons why Professional cyclists
wear those little finger less mitts.
Riders in the big races, like the
Tour de France are in the saddle for many hours every day and their
hand can get sore with constantly holding the handlebars, the palms of
the mitts are chamois leather or a synthetic equivalent to protect the
skin of their hands and with padding so as to take any vibration from
the road surface, in cobbled races like Paris to Roubaix this is very
important.
On the back of the thumb on most mitts there is a toweling section this
is very handy to mop any sweat or other liquids that may form on the
face or under the nose, for example. There are no fingers on the mitts
so that you can feel the brake and gear levers and for opening power
bars, liquid gels or the top on your feeding bottle.
Those are some of the reasons, but the main reason are to protect the
hands if you have a fall.
When you have the misfortune to crash
normally you put your hands out in front of you, its an automatic
reaction, if you land on the palms of your hands and lose skin its
impossible the grip the handlebars, which is very important.
If you cut your leg or arm, or ever break a collarbone you can ride on,
but not being able to hold the bars and that's your race over. If your
wearing mitts they should protect your hands, what they will not do is
stop you breaking your wrists.
So always wear your helmet and never forget your mitts!