Dispatch Server. The server that will eventually redirect you to a real NS. Its purpose is load balancing.
Notification Server. Used for authentication, notification, presence, requesting new SB sessions, etc.
Switchboard. The servers used for conversations. They work independently from the NS.
A client-initiated ping command (PNG) to keep the connection alive on connections that are automatically closed after a period of being idle.
Forward List. This is your main contact list. Principals in the FL can be in one or more groups.
Allow List. Contacts on your AL are able to see your status and invite you to conversations (switchboard sessions).
Block List. This list contains contacts that are blocked. They cannot see your status or invite you to conversations (switchboard sessions).
Reverse List. This list contains all contacts that have you on their FL.
Pending List. When someone first adds you, they will be added to this list.
Number identifying a particular contact list group.
Number identifying the current version of personal details, like contact list, BLP and GTC settings and phone numbers.
Additional information included in a command, separated by spaces.
Used to end commands and fields (\r\n).
Transaction ID, a unique ID (32-bit unsigned integer) coupled to a command sent to the server. The TrID is always the first parameter of a command. Its purpose is so that the client knows which command the server is replying to. Not all commands will have a TrID (such as PNG). Server-initiated commands will either have no TrID or a TrID of 0.
A server-initiated ping to verify whether the user is using an official client, and to check if the client is still alive. Not responding to a CHL or responding with the wrong answer will result in disconnection.
Microsoft's own client: MSN Messenger or Windows Live Messenger.