Cohen

WHAT IS A COHEN, LEVI AND ISRAEL?

When the Torah is read, a Cohen (or Kohen) is called to make the blessings over reading the first portion. Then a Levite is called for the second portion.  A Yisroel (Israelite) may then be called for the remaining portions to be read.  A Cohen is usually accorded greater honor or precedence before a Levite or an Israel.  What do these titles mean and how did they come about?