Spectator mode is a gamemode that allows the player to fly around and observe the world without interacting with it in any way.

Is it cheating to use spectator mode?

No it isn't. Cheating is only when you get an unfair advantage against other people, but since you can't have creative and spectator in multiplayer without an admin giving it to you, there's no other way you can use it against other players.

How do you get out of spectator mode?

Prior to the flattening, you can use /gamemode 3 and /gamemode sp to toggle spectator mode. That's now removed.