DIYbanter - Type of underpad on concrete

I am finishing my basement. I have a concrete floor. I will be
laying under pad on the concrete and carpet over the under pad. I was
told at home depot to buy only a red coloured under pad that has white
dots on it, because it acts as a vapour barrier as well as an under
pad. Does anyone know what this under pad is called ??
As well, when laying the under pad, do I have to staple it to the
concrete floor before laying the carpet or can I just lay it down and
lay the carpet right over it ?
Thanks.
From Ottawa, Ontario.