How does pedometer work

Hi
just oredred a omron pedometer from e-bay an awaiting delivery
got me thinking how do they work
I beleive you can hang them round your neck
Do you just wear them when go for a walk or is it all day
Whats the recommended number of steps and how does it measure that

Sure there an obvious answer but its evading me at moment

Cheers

James

Hi Denise
Thanks for that
What happens if you go in a car do you have to turn it off
Cheers
james