Identifer versus keyword

The terms "keyword" and "identifier" are not Java specific.

A keyword is a reserved word from the Java keyword list provide the compiler with instructions. As keywords are reserved, they cannot be used by the programmer for variable or method names.

Examples:

final
class
this
synchronized

Identifiers are the names of variables, methods, classes, packages and interfaces. They must be composed of letters, numbers, the underscore _ and the dollar sign $. Identifiers may only begin with a letter, the underscore or a dollar sign.

Examples:

int index;
String name;

index and name are valid identifiers here. int is a keyword.

A keyword cannot be used as an identifier.

Identifiers are names of variables. For example in

int a = 3;

a would the identifier. Keywords, on the other hand, are reserved (i.e. you can't name a variable with a keyword), pre-defined words that have a specific meaning in the language. For example in

if (a == 3)
    System.out.println("Hello World");

if is a keyword. It has a specific function and cannot be used as a variable name. Moreover, the words used to declare primitive types are all keywords as well, e.g. int, char, long, boolean etc. You can see a full list of Java keywords here

Keywords are reserved words like new,static,public,if,else,..

An identifier can be a name of any variable.

int age = 26;

"age" here is an identifier, while int is a reserved word.

The following example won't compile:

String static = "hello";
int public = 4;

you can't do this because "static" and "public" are keywords, that in this case are being used as identifiers, which is not allowed.