From the National Wildlife Foundation:
"Water acts different at the surface. Water molecules are attracted to each other, and like to stay together, especially on the surface where there is only air above. The attraction between water molecules creates tension and a very delicate membrane. Water striders walk on this membrane.
The secret of the water strider is its legs! The legs have tiny hairs that repel water and capture air. By repelling water, the tiny water striders stand on the water’s surface and the captured airs allows them to float and move easily." [There's a lot more at the link.]
The Fairfax County Public Schools website adds this (and some great photos):
Common Water Striders eat living and dead insects on the surface of the water. Some are aquatic (water) insects, such as mosquito larvae coming up from the bottom, and others are terrestrial (land) insects, such as butterflies or beetles that accidentally land on the surface.
Injured dragonflies are a favorite food, as are worms that fall in the water. Water striders have a sharp mouthpart, called a rostrum, to suck up body juices from prey.
The FCPS site also explains why the striders very quickly moved to the far side of the pond when I moved just a slight bit.
"Common Water Striders have very good vision and move quickly on the water."