The ATLAS detector is them big sibling of CMS, neatly placed in Point 1 across the LHC ring.

It has very similar physics program, but has chosen often very complementary detector design with respect to CMS choices. Characteric for ATLAS is the very large outer magnet system generating a toroidal field that bends particles along z; a much lighter outer structure (ATLAS would float on water, CMS would sink), and an electromagnetic calorimeter with a characteristic accordian structure. The downside of being so big is that one cannot see the whole detector when downstairs (so we stay on the surface); the most famous picture where you can still see all was taken during construction (see below).

The ATLAS Collaboration is also somewhat larger than CMS, but in many respects they are similar in purpose and structure. In the early nineties, particle physicists in Belgium wisely decided to put all eggs in one basket and chose to stick to one big future project at the LHC. That's why we now have no participation in ATLAS. The Netherlands did the opposite: they joined ATLAS instead of CMS.

The choice to build 2 experiments with similar purpose serves two purposes. On the one side, a discovery is a big claim to make, and – to quote Carl Sagan – extrordinary claims require extraordinary evidence. And this is exactly what happened with the Higgs boson discovery in 2012. In a spirit of friendly though fierce competition, the two expimerents presented the discovery together, after independently, and with very complementary techniques, the same underlying physics had been unearthed.

On the other side, it also is a more mundane insurance for things going really wrong. Since all these machines are their own prototypes, developing a lot of needed technology on the way, it requires half a miracle (no kidding, just amazing scientists and engineers) to make everything come together and actually work. Accidents do happen (and have happened, also with CMS!), so better to be prepared.

ATLAS has a neat website where you can explore more:
https://atlas.cern/Discover/Detector