Asterisk is a normal Linux application, not a distribution that you install from an ISO. It’s available from asterisk.org, but you have to install your own Linux (CentOS, Ubuntu, etc).
AsteriskNOW is a Linux distribution which includes Linux, Asterisk, and FreePBX (a separate project that provides a web GUI for configuration).
Many individuals use Asterisk itself on the Linux distribution they prefer and are familiar with.