Basically, the accelerator itself and therefore the particules are going through ATLAS. Collisions between particules (going in opposite directions) are setup to happen within the core of ATLAS. The objective is to build various layers of (different types of) detectors around this point of collision to search for specific particules with a very short lifetime generated by the collision.

http://atlas.web.cern.ch/Atlas/TCOORD/Activities/I nstallation/atlas_overview.gif

These differents layers of detection generate lots of data that is filtered to isolate a few 'interesting' events, which can then be processed by the computing Grid.

Romain.