(1)A declaration tag places a variable definition inside 
the body of the java servlet class. Static data members may 
be defined as well. Also inner classes should be defined 
here. 
Example:<%! int serverInstanceVariable = 1; %>

Declaration tags also allow methods to be defined.

<%!
  /** 
   * Converts the Object into a string or if
   * the Object is null, it returns the empty string.
   */
  public String toStringOrBlank( Object obj ){
    if(obj != null){
      return obj.toString();
    }
    return "";
  }
%>

(2)A scriptlet tag places the contained statements inside 
the _jspService() method of the java servlet class. 

<% int localStackBasedVariable = 1;
out.println(localStackBasedVariable); %>

(3)An expression tag places an expression to be evaluated 
inside the java servlet class. Expressions should not be 
terminated with a semi-colon . 

<%= "expanded inline data " + 1 %>

(4)Also we can use the following tag to give comments in 
jsp: 

<%-- give your comments here --%>