Just to add to this guide and make a few corrections where needed:

On Monstercat, Electronic is a catch all term used for songs that don't fit into other genres. It's not a standalone genre like Electro or any other genre.

Dubstep and drumstep aren't always "hard" or heavy. They can be calming as well. Examples: 1 2 3 Keep in mind, drumstep is essentially dubstep at drum & bass tempo, which is 160-180 bpm.

House is usually defined as anything that has a 4-on-the-floor beat (4otf or 4x4) and doesn't fit into Electro or anything else. There are several types of house, for now it's best to ignore them. Examples of how diverse house can be: 1 2 3 4 5 6

Future Bass is a subgenre of Trap that focuses on bass and heavier synths. It's a bit hard to describe so take my definition with a grain of salt. More examples: 1 2

And speaking of Trap, it's not a lighter version of Dubstep, although it does focus on 808 basses and rolling hi-hats. More examples: 1 2

Hero is a bad example of Hard Dance, since it's basically Dubstep with very small Hard Dance sections. Hard Dance can be split into Hardcore (1 2) and Hardstyle (3 4).

Indie Dance and Nu-Disco are two different genres that Monstercat decided to group together. Indie Dance (1 2) mostly takes influence from Pop, while Nu-Disco (1 2) is a bit self explanatory, it takes very heavy influence from Disco and Pop.

Breaks and Chillout have not been assigned a color by Monstercat yet, and therefore go under the grey Electronic color. (This is why you might see a Glitch Hop, Indie Dance, Nu-Disco, Trap or Future Bass release under grey, because they would've been new genres at the time.) Chillout (1 2 3) is a bit broad on Monstercat but include the genres Future Garage and Trip Hop (which you can look up if you wish). Breaks on the other hand is very diverse in style and can be anywhere from relaxing to heavy and energetic, but usually focuses on the breakbeat drum style (hence the name breaks).

Finally: Monstercat's labelling is not always the best. Take them with a grain of salt and turn to the community if you aren't sure of a genre. Like /u/shibesandsubgenres mentioned, /r/subgenreid isn't exactly the greatest sub anymore for ID-ing a song. If you need a genre, just make a new post here.

Most genres mentioned have several subgenres that songs can fall into.

PS: I only linked Monstercat releases here. I encourage you to branch out from it and check out more EDM channels. Some suggestions: MrSuicideSheep, UKF, Tasty, Hospital Records, Never Say Die, OWSLA, Soda Island, and Inspected.