A chariot archer is a mounted unit found in Civ 5. It can move four squares on open terrain, such as grassland or plains or snow, but can only go one at a time on hills, marshes, forests or snow.
It's advisable to have some melee units around when there are a lot of enemy units around, since these units have ranged combat strength of 6, but melee of only 3 so are very vunerable to melee combat.
These are rendered obsolete by a knight.