Why are there so many erasers to choose from? Well they all have a different purpose. The basic two would be a vinyl eraser and a kneadable eraser. But everyone know that you really can’t have too many art materials, don’t they? It can’t be just me!
Kneadable Erasers – to lift of by patting or rolling.
Kneadable erasers come in different colours but I like the grey ones from Faber-Castell.
Cutting them in 9 pieces, I keep them in a small container so they stay clean and one piece will last ages.
To activate, stretch and compress (repeat until soft and tacky). This eraser should be rolled over the surface you wish to clean off. Clean by activating again (magic).
Vinyl Erasers – to take off by pressing the eraser into the paper and rubbing. 
This is the most common kind of eraser that people think of and use when writing and drawing.
Vinyl erasers come in all colours, shapes and sizes. I prefer the white, medium hardness ones from Faber-Castell.
I cut them into 3 pieces as I often use the edges to draw hair or grass with. I also keep my vinyl eraser pieces in a small container which keeps them together and clean.
Vinyl erasers don’t need to be activated but when they get grubby, clean them by rubbing the dirty bits on scrap paper.
Mechanical Erasers – long vinyl cylinders in a holder for controlled rubbing out.
There are many different holders for mechanical erasers and all different sizes. The thinner the vinyl is, the harder it is, this is so that it holds it’s shape. Like mechanical pencils, the size of the insert must match the size of the holder.
These erasers can give you better control as they are handled like a pencil.
You can find eraser guards in some art stores which are very thin metal sheets with shapes cut out of them. These are useful here to do small dot highlights or tips of grass – they give a perfect edge to your erasing.
Battery Operated Erasers – high speed rotation erases cleanly and fast. 
There are a few different brands on the market and some of them are quite expensive. Some come with extra eraser tips, some with batteries. Personally I have found that the cheap ones last just as long as the more expensive art brands.
One of the things to check is the firmness of the actual gripping part. If this is too lax or the eraser part sticks out too far you will have a very wobbly end which is hard to control (as all wobbly ends are).
Make sure to keep all your left over bits of eraser, you can use them to boost your shorter bits for longer use if, like me, you object to wasting half of each tip.