craftgrrl | Knifty Knitter Loom Question

I am a newbie to knitting and was advised to try knifty knitter looms. I bought a pack of circular ones, which I believe are for hats and scarves? (the package didn't say much). I also noticed tons on long vertical looms- does anyone know what these are for? I'd like to be familiar with their products so I can get what I need...their website wasn't much help either. Also, for beginners, do these work really well? I want something super easy. Finally, I have no clue how much yarn to buy for a hat or scarf..the employees at the craft store couldn't help me either. Does anyone know of any basic guidelines for how much yarn is needed for adult hats/scarves/etc.?

Thank you SO very much!