execute query() used in select operation.
execute update() is used in delete, update and insert 
operations.

execute query() returns a single resultset object.
exceute update() excecutes the insert, update and the delete
operation of the sql statement.This returns the integer
value or say the number of record affected by the SQL
statement execution.

executeQuery() is used to perform DDL operation like 
(create,alter) while executeUpdate() is used to perform DML 
operation like (insert/update/delete).

excuteQuery() its method used perform select query and 
delete query return ResultSet object.
 excuteUpdate() used to perform DML operation return type 
int

Execute query():it is using for DML(Data definition
language) and DDL (Data Manipulation Language)and mainly its
used for select query 
example:String sql="select * from Employee_Details";
        st.executequery(sql);

Execute update():it is using for DML(Data definition
language) and DDL (Data Manipulation Language)and mainly its
used for insert query,update query,delete
example:String sql="insert into Employee_Detail........";

        st.executeUpdate(sql);