Identifiers in Java Programming are the names of variables,methods,members,classes,packages and interfaces .identifiers are just the way of referring all these . 
identifiers must be composed of letters,numbers and under scores and the dollar sign.identifiers are case sensitive. so hello is not same as Hello.

 identifiers must start with a number ,letter,under score or a dollar sign.
some valid identifiers are :
$name
_pet
animal
11123class