ATLAS is designed as a general-purpose detector. When the proton beams produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies may be produced. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that, whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. Experiments at earlier colliders, such as the Tevatron and Large Electron-Positron Collider, were designed based on a similar philosophy. However, the unique challenges of the Large Hadron Collider—its unprecedented energy and extremely high rate of collisions—require ATLAS to be larger and more complex than any detector ever built.

The ATLAS detector is made up of a number of different sub-detectors each of which serve a different purpose:

• The Inner Detector is a silicon detector used for tracking charged particles. It determines the direction and momentum of the particles.
• The Electromagnetic Calorimeter determines the energy of photons and electrons.
• The Hadronic Calorimeter determines the energy of hardrons (protons neutrons etc...).
• The Muon Spectrometer determins the momentum of muons.

To get an idea of how each part fits into the whole detector ATLAS has made the video:

• ATLAS - Episode 1 - A New Hope

To get an idea of how each sub-detector functions ATLAS has made the sequel:

• ATLAS - Episode 2 - The Particles Strike Back
• Or you can visit the ATLAS Detector Overview where Episode 2 has been split up into separate sections for each detector modeule.