Lobster Gloves, The Basics
Lobster gloves are designed to keep your hands warm in cold temperatures without limiting your dexterity like mittens do. Lobster gloves can be thought of as a cross between regular gloves and mittens.
You will see that lobster gloves are like mittens split down the middle - one pocket for your first two fingers and one pocket for your last two fingers (along with one for your thumb.) Keeping your fingers together increases warmth while the split-finger design allows you to perform basic tasks without removing the gloves.
They are great for cyclists because we deal with very cold temperatures on our hands, yet we still need to brake and shift to ride safely. Normal gloves don't keep us warm enough, and mittens don't let us shift very well. That's why lobster gloves are so great.
(For extreme cold, also consider pogies.)