The A or P refers to the lens diameter/focaL length. Quote:
Cokin A Series Filters Recommended for lenses between 36 and 62mm in diameter with a focal length over 35mm.
Cokin P Series Filters For shorter focal lengths 20-24 or 28mm as well as for wide angle zooms (24-50mm / 28-85mm / 35-135mm) the Cokin P System is strongly recommended.
Have a look at the Cokin site where it's all explained.