I have read somewhere someone mentioning "background" sprites. What are these and how do I use them ?

The terminology "background" sprite is in reference to a virtual sprite, not a physical sprite. It is a software technique that is used that makes it appear that you are using sprite hardware but in reality you are not.

This was a popular technique that was used in such games as Mortal Kombat 2 in order to draw the actors. It involves writing directly into the background map. You can even do smooth, single pixel scrolls of "background" sprites that contain transparent pixels if your code is advanced enough to handle that.

Since the release of the GBC, this technique is not as feasible to implement due to limitations of the GBC background colo(u)r palettes.