Anything related to the usage of commands or command blocks in the game Minecraft. If you're building a redstone device without commands, use the minecraft-redstone tag instead.
Commands are functions that are carried out by the server by typing a string following a slash (/) in the chat box in the game Minecraft. They can also be executed by a Command Block.
Commands can be used by server administrators and operators for maintenance and customization, by map makers for creating custom games, or by anyone else who wants to do something interesting with their game.
This tag is just about commands, command blocks, function files, and their usage. If your question is about a specific Bukkit plugin (and your problem doesn't involve a crash), use minecraft-bukkit. If your question is about redstone devices, use the minecraft-redstone tag. If your question is about the interaction between redstone (other than just a clock) and command blocks, use both.