Sadly, Mojang removed the page that explains what the service status is/was.

So, here goes:

• Auth is authentication services - the stuff you need to log in.
• Session is the part that handles game sessions and connection to game servers.
• Skins is the skins service. Without it, everyone is Steve.
• API is the rest of the Mojang API in general.

And the status goes:

• Green - good, probably.
• Yellow - we don't know really what it means, but it's worse than Green.
• Red - it's probably broken.