File used by Adobe Digital Editions, a program used to manage and read eBooks as well as other digital publications; contains data to activate and download an eBook but does not contain the eBook itself; used to authorize, legally download, and read purchased eBooks in Digital Editions.
ACSM files are served to Digital Editions by Adobe Content Server. They can be opened directly in Digital Editions.
NOTE: Even if you have a different eReader software other than Digital Editions, such as Kobo Desktop, you may still receive an ACSM file when downloading an eBook. However, you still must install Adobe Digital Editions to download the eBook and then it can be read and transferred to your eReader device.