Guilds and Shards Outside of Ravnica and Alara
The Ravnica block and the Alara block introduced a memorable shorthand into the game of Magic for certain combinations of colors. But there has always been a slight controversy about how to use these names.
Ravnica introduced the ten Guilds, which are combinations of two colors:
Selesnya: White and Green
Alara introduced the five Shards, which are three color combinations of friendly colors:
Bant: White with friendly colors Green and Blue
Esper: Blue with friendly colors White and Black
Grixis: Black with friendly colors Blue and Red
Jund: Red with friendly colors Black and Green
Naya: Green with friendly colors Red and White
The slight controversy is brought up by Kiora here: She's not really Simic. Each Guild and Shard not only has a color combination, but a flavorful and mechanical identity. Kiora Atua is Green and Blue, but she doesn't fit the identity of the Simic, so it's a little awkward to call her that. Simic loves to start with small creatures and build them up, but Kiora is mysterious and reverent towards huge natural creatures. Mechanically, Simic loves +1/+1 counters and changing a creature's stats temporarily, but Kiora just ramps, ramps, ramps like a mudruther fudrucker until she can hit you with a Lorthos or Kozilek.
Using the Guilds and Shards as shorthand for color combinations, ignoring the mechanics of the actual Guilds and Shards, is so convenient that everyone does it. But there will always be someone who says, "That's not an Esper deck! It has no artifacts!" or "That's not a Boros deck! It doesn't have small aggressive creatures!"
I think it's a good idea to keep the difference of Guild/Shard identity and color combination in mind. I try to say, "Kiora is a Simic-colored planeswalker" and "Oloro, Ageless Ascetic is an Esper-colored legend" whenever it could be confusing. But most of the time no one cares and I don't worry about it.
P.S. There are five other three-color combinations, called "wedges", which have informal names:
Rug: Red Blue Green (Blue is always shortened to U to avoid confusion with Black)
Junk: Green White Black (jokingly named after Jund)
American: Red White Blue (or British or French, depending on whether you happen to be from America, Britain, or France)
And the last one is not as common. I've heard Red White Black called Oros by Luis Scott-Vargas, after Oros, the Avenger. It's most often associated with Kaalia of the Vast in Commander, so I sometimes call it "Kaalia colors" and I've called it "newspaper colors" before- black and white and read all over.
EDIT: I've also heard the Red White Black combination called "Borzhov", Boros plus Orzhov. That seems to work, especially if your deck has Boros and Orzhov themes.
ANOTHER EDIT: I just heard about the 'Volver cycle in a roundabout way from Maro's blog. The Apocalypse set had several cycles that used wedge colors, and they used distinctive names.
Dega: White with enemy colors Black and Red
Ceta: Blue with enemy colors Red and Green
Necra: Black with enemy colors Green and White
Raka: Red with enemy colors White and Blue
Ana: Green with enemy colors Blue and Black