What is the difference between <> and != operators in MySQL?

No difference. <> is sql standard, != non-standard.

<> should be preferred, all things being equal, since it accords with the sql standard and is technically more portable...

!= is non-standard, but most db's implement it.

sql:2008 grammar:

<not equals operator> ::=
  <>

They are both exactly the same. See the documentation.

http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_not-equal