The simplest way to express the difference between mending and alterations is this: if you mend something you're returning it to the way it once was. If you're making an alteration then you're changing it in some way but you still have the basic garment. (If you want to turn it into something completely different that's called refashioning.)
So if your favorite jeans have a tear in the knee you can...
Learn how to fix the tear in my Basic Mending class at the Sew-op on September 27th from 10-12. Or...
Learn how to alter the jeans into cut-offs in Rebecca's Alterations class at the Sew-op on September 29th from 5:30-7:30.
Hope to see you there!