Big room house is a sub-genre of electro house. It's popularity soared in the mid-2010s, when popular artists began using them in all their songs.
The genre first appeared early 2010, being pioneered by artists Swedish House Mafia & Dada Life. It began to find its place at music festivals like Tomorrowland, Ultra, and EDC. It became prominent to many popular artists, and is heard from artists like will.i.am and Britney Spears. It became internationally prominent around 2013, with tracks like Animals by Martin Garrix and Levels by Avicii topping radio charts everywhere, and extending beyond the EDM scene.
The genre generally has a BPM of around 126-132. It uses lengthy techno build-ups, and powerful electro drops. Big room sometimes uses 4/4 hardstyle kicks. A typical big room uses heavy kicks, minimal musical elements, and once in a while just a syncopated synth/percussion. It's strength lies in its minimalism. It usually uses one bassline with added highs or lows. The beat is usually with just a tom/kick, maybe a hi-hat/clap.
The structure of big room house is similar to that of American progressive house music from the late 00's. The typical structure are two complete build-ups with breaks, two drop sections, maybe one or two breakdowns, one which may or may not be the intro/outro sequence.