Midcap technically can be considered as counters that have a middle order market capital, say Polaris Software, Visual Software.
Big caps on the other hand are counters like Reliance, Infosys that have a big portfolio.
Small caps are generally counters(eg:California Softwares,Diamond Cables) considered a little above penny stocks(eg: Skylid tele, Sun Beam)
But generally small caps and penny counters are used alternately.
I hope this clears a bit of your doubt.
Do get back if you have any queries.