The underlying implementation that identifies the environment your Addon is running on. For a list of platforms, refer to Supported Platforms.
One of the two physical pieces of software that make up the game:
Server: the piece of software that is solely responsible for running a game and accepting connections from players. This is where your addon will be stored. It is commonly known as the "server".
Client: the piece of software that allows you to play the game on your machine, either in Singleplayer or Multiplayer when connecting to a server. This is the software to which your addon will be sent to. It is commonly referred to as the "client".
One of the two logical parts in which Minecraft is separated:
Server: the part which controls the world and all the entities in it, handling the main logic of the game. Note that this does not correspond to the common meaning of "server", which usually indicats the dedicated server distribution.
Client: the part that renders the world on a screen and receives input from the player, which will then be passed on to the server. It effectively corresponds to the "what you see" part when you play the game.
A mod made for loading with the Sandbox Mod Loader and that interacts solely with the API exposed by it. The mod is usually hosted on Dedicated Servers and sent to Clients upon login, but nothing forbids an Addon being installed directly on a Client, maybe for usage in a Singleplayer world.
The piece of software responsible for loading mods into the game. In the case of Sandbox, that is the Sandbox Mod Loader, which is built into a platform.
Part of the Sandbox API a developer can choose to interact with. Refer to the API Modules Parity page for a list of available modules and their availability on target platforms.