You buy an ACSI book which lists all the sites taking part (a bit like the Camping Cheque guide, but less detail about each site) - and sites cost 11, 13 or 15 euros. Dates are limited to 'low season' just like Camping Cheques - so generally not mid-July to mid-August, but some up until early July and some from mid-August onwards. Inside the book is a detachable card which you remove and show to the site reception when booking in. Just like Camping Cheques there are some special offers - like 7 nights for 6, or 14 nights for 11. Prices may rise next year - as may Camping Cheques.
We generally use both - but this year didn't stay on any ACSI sites, and last year discovered one site that we decided not to stay on - but usually sites are fine.
The 2010 book can be ordered from www.campingcard.com - usually around Christmas/New Year time.