Lets say you were just an ordinary foot soldier and you come up against a group of heavily armored cavalry that is bearing down on you. What was that experience like? Did most of the casualties come from the mass of the horses crashing into people, or by the riders of the horses? Do we have any sources that describe what this was like?
Do you have a period you specifically want to know about? This is ranges from slightly different to totally different if we're talking about Asia Minor and Syria circa 333 B.C., Italy in 216 B.C., Gaul 500 A.D., Palestine 1066 A.D., Brandenburg in 1630/31 A.D., Silesia in 1740 A.D. or Sedan in 1870. I'm sure we could whip up some kind of generalized answer but I feel by better knowing what you're thinking about when you ask this question I (or someone else) could provide a more in depth, specific answer tailored to you.