If I understand your question correctly, the difference is fairly straight forward. 

Here it is:

• A Use Case diagram shows actors and use cases as well as relationships between actors and use cases.
• One type of use case to use case relationship is the <<extend>> relationship - this signifies that one use cases  is extended by referencing another use case.
• NOTE: The extend relationship can be shown in the use case diagram but an alternate flow cannot be seen in the use case diagram.  This is one distinction.
• For each use case in the use case diagram, there will be a use case specification which contains the details of the given use case.
• One of the key constructs of the use case specification is the "Main Flow" which is a step by step description of the behaviour of the use case.
• The use case steps show the actor/system (business or IT) interactions.
• One of the system steps can be an extension point. => this is a way of saying that is the place in the main flow where the other use case (extending use case) is referenced.  From a flow perspective the next step to takes place is the 1st step in the referenced use case.
• An alternate flow is a set of "alternate" steps which are part of the same use case (as opposed to steps in another extending  use case).  Generally, the alternate flow begins by stating which step in the main flow is the step where the alternative starts.

Also, note that an alternate flow in one use case could also contain an extension point which references another extending use case.

Hope this helps!

Adrian

 Thank you Kimbo and Adrian! 

@ Kimbo

when you mention different ways of doing the same thing..what does that "difference" means? I originally thought an AF is another set of steps that will get you to the successful post-condition ( ex. you can get coffee from the drive thru or walk in to the actual store and buy the coffee) either way you get the coffee. But I think my interpretation is wrong and that's where my confusion started. 

Let's take submitting this message that I'm writing as an example...

Usecase name: submit message

step 1. type message

2. click submit

Can  choosing the text box option, checking spelling  and other formatting tools be considered as an AF? And can preview and cancel be extension points?

Thank you again!