what is the difference between erasing RW DVD and finalising a RE DVD?

Answer 1

Erasing a DVD means you are wiping it clean and erasing all data on the CD or DVD.

Finalizing a DVD or CD means after it burns the movie or data or music onto the disc, then it has to finalize it at the end so that it "wraps everything up" into a nice package. Its just the final thing it does before you are allowed to use it.