ACM SIGGRAPH is an interdisciplinary community made up of thousands of people who share a passion and interest in the research, technology and applications in computer graphics and interactive techniques. It stands for the Association of Computing Machinery's Special Interest Group on Computer Graphics and Interactive Techniques.
Whereas SIGGRAPH and SIGGRAPH Asia are the annual conferences owned and sponsored by ACM SIGGRAPH! This year is SIGGRAPH Conferences' 44th edition held in Los Angeles, USA, and SIGGRAPH Asia's 10th edition held in Bangkok, Thailand.
Read more here: http://www.siggraph.org/about/what-is-acm-siggraph