The ACM (Association for Computing Machinery) is an international professional society (similar to the IEEE or IPENZ, for example). You might want to check the ACM Website for a detailed description of what the ACM provide; a brief description is that they run a digital library, sponsor conferences, that kind of thing.
The ACM like running student chapters in Universities, in order to increase their membership. WaikatoUniversity have one of these see Waikato's chapter page; basically, it's a group similar in scope to the WaikatoLinuxUsersGroup, but with a general CS focus. It can do whatever it wants; previously they've had talks from a couple of CS professors and looked after the organisation of a graduate students' roadshow of NZ.
There are chapters in heaps of other universities; in NZ, there's Auckland, Massey, Victoria and Canterbury.