Niche vs Unique

[Bigc779]

In real simple terms could someone please explain the difference if there is one, between Niche traffic and Unique traffic I read about these terms all the time. Which one would be better to purchase if I were to do so.
Thank You again for your help
I would also like to thank you for the help on my last post.

[martinsc]

niche traffic means that the surfer is looking for a specific niche (teens, hardcore, milf...)
unique means unique visitors...