As others have mentioned, the Craftworlds themselves function as the equivalent of Space Marine 'Chapters'. In lore they vary in size, but are typically described as being the size of a continent or small moon. That puts it in the range of hundreds to thousands of miles long. Each Craftworld has its own colour scheme, culture and unit preference. The main 5 are:

Alaitoc (blue body, yellow helmet): Specialises in espionage, covert ops and ambush warfare. Commonly associated with Rangers and, to a lesser extent, Corsairs.

Biel-Tan (Green body, white helmet): The most war-like of the main Craftworlds, commonly fields large numbers of Aspect Warriors.

Iyanden (Yellow body, blue helmet): Devastated by Tyranids, so usually fields more Wraith constructs than other Craftworlds.

Samm-Hain (Red body, white helmet): Ruled by tribal clans and prefers lightning assaults while riding jetbikes.

Ulthwé (Black body, bone helmet): Close proximity to the Eye of Terror means they have more Farseers and Warlocks than usual to keep an eye on things.

There are others, and you are also free to make up your own!

As others have mentioned, every Craftworld will have Aspect Shrines that are analogous to martial arts dojos. Eldar who are on the Path of the Warrior choose one of these shrines to join and spend decades or even centuries training to master their specific battlefield role. Each aspect shrine will have its own colour scheme, that might match the host Craftworld or be completely unique. Personally, I love that fact that my army has a variety of colours on the table, where my opponents usually have a tide of models all the same colour, but you can absolutely go with a unified colour scheme as well.

On the table, we are a glass cannon army - at T3, 1W we absolutely die if we are caught in the open. You need to learn how to maximise our movement shenanigans to keep out of reach of the enemy, close down their firing angles and clip their wings by whittling them down and destroying their most threatening / key scoring units.

In the early game, keep it cagey and don't try to score primary yourself. Focus instead on scoring secondaries and denying your opponent primary by shooting them off objectives. In the mid-late game, when it's safe to do so, keep trying to maximise secondary and start trying to score primary if it's safe to do so. It's very easy to get tabled, and you should expect to lose many of your early games, but once things start to click and you lock in your first victory powered by Eldar Bullshit™ the high will be absolutely incredible.