In the Photon Cloud, the Application ID (AppId) is the main identifier for a Title and it's Virtual Application

A (low level) command to acknowledge reliable commands. Used for Reliable UDP (RUDP).

Players in a room are also called "Actor". An actor has an ActorNumber, also referred to as actorId

Applications contain the game logic, written in C# and run by the Photon Core. They extend the abstract class Application for this. The Photon Cloud makes use of Virtual Applications

A batch file or MsBuild project to compile and copy applications for their deployment. Takes care of getting a clean build for deployment.

A game's logic, which runs on top of Photon Core. This "layer" handles operations and events.

Can refer to either low-level channels in the RUDP protocol or to the "conversation" channels in Photon Chat

Photon Chat is a lightweight service for communication between users. Users can join channels, send private messages and make use of a user status which is available for friends. Chat uses a separate connection and is independent of rooms.

Applications that connect to a server are called clients. In Photon's case, they initiate a connection to a server, which enables them to message other clients. Photon clients are programmed with a client API, also referred to as client library.

Commands are used on the eNet protocol layer to transport your data or establish / shutdown connections. You won't write these, but we will explain them for your background knowledge.