This is a list of terms that you should know if you want to use D++ (or any other Discord library). These terms are not D++ specific, and are commonly used throughout most of the Discord developer community. This list, with a few exceptions, is Discord specific, that is to say that this is not an explanation of commonly used C++ terms, it is for people who are not familiar with the terminology of the Discord API and libraries themselves.

Glossary

Listed in alphabetical order, with terms in bold, here are the basics on...

Action row: A collection of up to five components which is attached to a message.
Application Command: See slashcommand.
Audit log: A log of events that have happened in a guild.
Auto mod: Discord's low-code solution to moderation. However, it is very limited in scope.
Badge: A decoration on someone's profile showing certain things about them, such as if they have Nitro, if they are a Discord developer, etc.
Bot token: A secret string of characters that is used as a "login" to your bot. If you lose it or it gets leaked you will have to get a new one from the Discord developer portal, so be sure to keep it in a place that is both secure and where you won't forget it.
Button: A component on a message that can be styled that sends an event when clicked on by a user.
Cache: A type of storage efficient for things like messages.
Callback: While not strictly related to Discord, it is used a LOT in D++. A callback is when a function is passed to another function, sort of like how you might give someone a telephone number (you give them the means to do some sort of interaction rather than asking them how to interact), which is used to handle responses to events.
Cluster: A singular bot application, which is composed of one or more shards, a cluster is the centre of bot development.
Component: A component is anything that can appear in a bot's message besides text, such as buttons and select menus.
Drop down/Select menu: A component of a message that upon being clicked drops down and allows the user to select an option.
Embeds: A widget attached to a message which can contain multiple fields of texts, an image, and much more information.
Ephemeral: A message only visible to the user being replied to.
Event: Something that a Discord bot can respond to, such as a message being sent, a button being clicked, or an option being selected, among others.
Guild: What the Discord API (and most libraries for it) call a server.
Intents: The right for a bot to receive certain data from the Discord API.
Interaction: An object that contains information about whenever a user interacts with an application, such as sending a message or clicking a button. It is the main part of an event that will be accessed in an application.
Modal: A pop up form that contains text that can be sent by a bot.
Shards: A shard manages part of the workload of your Discord application.
Slashcommand: The primary way a user interacts with a bot. It is a command sent to the bot with parameters (which may be optional) and is initiated by beginning a message with /.
Snowflake: An unsigned 64 bit integer (it can represent anything from 0 to 2^64-1) that is used by Discord to identify basically everything, including but not limited to, guilds, users, messages, and much more.
Subcommands: A command which is derived from a different command, such as a bot that allows a person to get statistics for Discord might have a stats guild command and a stats global command, both of which are subcommands of stats.