Coming off of the much longer article on the basics of crafting, this article will be significantly lighter reading. In this article we will cover the various domains, their advantage/weakness against each other, and why you should build teams of various domains.
So, what even are domains?
There are six domains in Guild of Guardians and each character belongs to one of them. According to the Guild of Guardians White Paper, “Units of a particular domain gain advantages against those of a 'weaker' domain, which adds strategic depth to the game.”
Originally, domains were referred to as elements. When the elements were converted to domains, the names changed:
Fire became Cinder
Water became Hydris
Earth became Terra
Dark became Umbrus
Light became Solas
Domain advantages:

From the above, we can see that Cinder is strong against Terra, Terra is strong against Hydris, and Hydris is strong against Cinder. Anyone familiar with elemental advantages in other games will find this to be pretty standard.
Umbrus and Solas are strong against each other.
From the in-game chart, we can see what that advantage provides. Having a domain advantage provides an additional 25% damage boost against restrained Guardians and also provides an extra 25% boost to skill energy. This is a significant boost that should not be ignored.
As of right now, there are three game maps open to play: Requiem, Brimstone, and Deadlands.
Your enemies in Requiem will be Umbrus; in Brimstone they will be Cinder; in the Deadlands they are Hydris.
To maximize your effectiveness in these dungeons, you will want to utilize heroes of the stronger domain. So, in Requiem, the earliest part of the game, the domain is Umbus. Since Umbrus is both strong and weak against Solas, the two domains will each benefit from fighting the other. Objectively, on Requiem, build whatever team suits you.
In Brimstone, the enemies will be Cinder. So, to maximize your effectiveness there, you will want to use Hydris characters. Finally, in the Deadlands, the domain is Hydris, and Terra units will be the most effective.
Fortunately, when you want to change out your team, you can reset the character you are dropping and receive the resources you spent on them back. You can then apply those resources to the character you want to add to your party.
Guild of Guardians rewards strategic planning and gaming. Domains add a deep strategic element (pun intended) to gameplay and are something that should remain top of mind if you want to advance.
And as always,
See you in Elderym!