Dosto mera sawal ye hai k "cache" kya hai?

ager hum market se computer lene jayein to dukaandar cache k bare me btata hay k iski cache speed flaan flaan hai..
ap muje plz detail men bata deinge k computer me cache memory kya hoti hai??
aur kitni zyda ho to zyda acha rehta hai computer performance mein?

aur ki kis kis size me available hoti hai?