int executeUpdate() Executes the SQL statement in this PreparedStatement object, which must be an SQL INSERT, UPDATE or DELETE statement; or an SQL statement that returns nothing, such as a DDL statement.
executeQuery() --- This is used generally for reading the content of the database. The output will be in the form of ResultSet. Generally SELECT statement is used.
executeUpdate() --- This is generally used for altering the databases. Generally DROP TABLE or DATABASE, INSERT into TABLE, UPDATE TABLE, DELETE from TABLE statements will be used in this. The output will be in the form of int. This int value denotes the number of rows affected by the query.
execute() --- If you dont know which method to be used for executing SQL statements, this method can be used. This will return a boolean. TRUE indicates the result is a ResultSet and FALSE indicates it has the int value which denotes number of rows affected by the query.