Gaiters are designed to protect the top of your boots from unexpected puddle plunges and keep your trousers out of the way of wet grass, heather, mud and crampons.
Look for a snug fit that still allows for a little flex as you move (Don't restrict those calves!).
With waterproof membranes and tough fabrics, gaiters allow an extra defense against the elements without the restriction and discomfort of overtrousers.
Gaiters are also great at keeping unwanted gravel and debris out of your boots and shoes, look for a low cut, flexible option for lightweight protection. Longer cut, heavier-weight gaiters are more suited to wet weather protection.