USA This is normal. Until the 1970s, metal boxes were used for switches and receptacles in the walls. The ground wire was attached to the box, and the metal strap holding the switch to the box provided a ground for the switch. Then in the 1970s, plastic boxes were invented. You cannot ground a plastic box, and if the box is not grounded, the switch cannot be grounded. So, they added a ground screw to the switches. If you install switches in plastic boxes, they need to have the bare ground wire hooked to the green ground screw at the top/bottom of the switch. If the switch is in a metal box, the metal strap will ground the switch to the box and you don't have to do anything special.
As far as the switches having two screws or holes for wires, this is normal for single pole switches, where you can turn the light on from only one switch. Sometimes, there will be two black wires and sometimes one black and one white. This is also normal, and nothing to worry about.
If you are replacing switches, make sure you note how the existing switches are hooked up, and hook up the new ones the same way. If you are not 100% sure how to do it, please call a licensed electrician.