Info about acsm can be found here:
http://www.ereadable.com/info/FAQ.asp
and here
http://www.adobe.com/products/digitaleditions/faq/
Basically, acsm files open in Adobe Digital Editions.
If you have a Sony PRS 500 then you are out of luck. But the Sony PRS 505 will read them.
And welcome to the forum, papa4ahe.