Beamos are stationary sentry turrets that scan the rooms they're installed in with their single eye. Once they spot movement, they'll fire a powerful beam at whatever has wandered into their sights. You can send this beam right back at them with a Shield Bash from your Hylian Shield, which will temporarily stun it. That's your cue to run in close and slice it along the glowing seams of energy that hold it together. Once you've eliminated all but the primary segment, attack its eye to defeat it.
Find out more at Zeldapedia: Beamos.