Troj_Dloader.ACS Trojan arrives on a system as an attachment to email messages spammed by another malware or a malicious user.

It sends HTTP requests to certain IP addresses to download and execute files. Trend Micro detects the downloaded files as TROJ_CUTWAIL.BC and TROJ_PANDEX.CHL. As a result, routines of the downloaded Trojans are also exhibited on the affected system.

Technical details can be found at this Trend Micro page.