If one usecase must use another usecase, then use include
relationship. If one use case may use another usecase, then
use extends relationship. uses relationship is defined in
UML 1.1.

Includes:

if(TRUE)

   include the use case

Extends:

if(condition)

   extend the use case

It's a matter of unconditional VS. conditional insertion

(Source : Uml 2002--The Unified Modeling Language
 By Jean-Marc Jézéquel, Heinrich Hussmann, Stephen Cook)

Uses:

Defined in UML 1.1

The <<include>> relationship means that the flow of that 
use case is always included.
But a <<extend>> relationship means that the flow of the 
extending use case is only
included under specific conditions, which must be specified 
as the extension point of the
use case being extended.

"X uses Y" indicates that the task "X" has a subtask "Y"; 
that is, in the process of completing task "X", task "Y" 
will be completed at least once. 

"X extends Y" indecates that "X" is a task of the same type 
as "Y", but "X" is a special, more specific case of 
doing "Y". That is, doing X is a lot like doing Y, but X 
has a few extra processes to it that go above and beyond 
the things that must be done in order to complete Y.