While ice itself is slick, slippery, and difficult to navigate across under most circumstances, skaters easily glide across the ice.
Imagine there’s a large, flat sheet of ice out in front of you, and someone unceremoniously shoves you across it at a high speed. What are you to do? If you’re wearing conventional shoes, without crampons or blades attached to them you’re going to have a difficult time. Ice is a very low-friction surface, and there’s very little you’re going to be able to do to change your momentum without slipping and perhaps falling down. You’re bound to simply slide along until either you run into an obstacle or slowly come to rest, likely a long way from where you began.
But if you put thin blades on the bottoms of your shoes — e.g., wear ice skates — you’ll discover that the situation is very much different in this case. As long as you can remain on your feet, with only your blades touching the ice, you’ll find that you can control your motion relatively easily simply by applying forces through your feet (and the blades) to the ice down below. You can speed up, slow down, or change direction at will, and only if you fall or lose control of your skates (and body) will you wind up in a similar situation to the no ice skate case. It might seem miraculous, but there’s physics behind what you’re experiencing at each and every step. Here’s how it all works.
Here on the surface of Earth, at normal atmospheric pressure and wherever you have sub-freezing temperatures, almost all of the ice you’ll encounter comes in a very specific configuration: normal hexagonal crystalline ice, sometimes known as Ice Ih. Ice, just like water, is made up of primarily a very simple molecule (H2O) with two hydrogen atoms anchored by a single oxygen atom, and with a very specific bond angle between them. Whereas in liquid water, the bond angle is 104.5° between the connecting lines of each O-H bond, in normal hexagonal crystalline ice, the bond is lengthened into the shape of a more perfect tetrahedron: 109.5°.