Friction is caused by forces between two things when they touch, like when you drag your foot along the sidewalk and feel and hear the surfaces scuffing together. It doesn't matter how smooth things look or feel—peer at them under a microscope and you'll see they're always rougher in close up. When you drag your foot on the ground, the jagged bits on the two surfaces snag against each other, slowing you down and stealing your energy. The rougher the touching surfaces, the more friction there is, the harder you have to work, and the more energy you waste.
Two people slide down an icy hill on a sledge.
Friction also happens because of a microscopic "gluing" effect when two things touch. When a gecko climbs a wall, millions of tiny hairs on the base of its feet are touching the wall at the same time. Molecules on the tips of each hair attract to molecules on the outer surface of the wall with a microscopic amount of what's called van der Waals force—a really weak kind of electromagnetism. Although each hair provides only a smidgen of magnetic force, there are so many hairs on the gecko's foot that, together, they can provide enough force to support its weight. Read more about this in our article on adhesives.
Photo: What makes ice slippery? Scientists used to explain it this way: as you move over the ice, you compress it and make it melt, creating a smooth, wet layer that acts as a lubricant, helping you slide over the otherwise rough surface. Unfortunately, that attractively simple explanation has now been called into question—and one current theory is that ice is slippery because it has a permanent lubricant coating that grows thicker or thinner according to the temperature. Whatever the answer, a question like "Why is ice slippery?" can be explained in many different ways, at many different levels of complexity, as Richard Feynman points out in this excellent video. Photo by Chantel M. Clayton courtesy of US Navy.