A small doubt in flash cs3

Dear Friends,
I am having a small doubt in flash cs3.
I am trying to display a simple “trace” statement from an .as file and it was throwing an error called "5000: The class ‘class1’ must subclass ‘flash.display.MovieClip’ since it is linked to a library symbol of that type "
i am having only 1 fla named - document_Class.fla
and 1 .as file named - class1.as
this is the simple code i writen for class1.as file ie
package {
public class class1 {
public function class1():void {
trace(“class1”)
}
}
}
inside fla ther is nothing…
but when am writing the code as,
package {
import flash.display.MovieClip
public class class1 extends MovieClip{
public function class1():void {
trace(“class1”)
}
}
}
its working fine… wats the difference with both the codes and why i have to import movieclip if am not using is my ques ??
waiting for ur replies.
Rajesh