Commands
Slash commands (like /hello) that a member types directly in Discord. Each command has its own visual logic defining exactly what happens when someone runs it, reply with a message, look something up, update a score, whatever you build. A command can also have subcommands and subcommand groups (like /settings profile edit), autocomplete suggestions on a text option, and its own name and description in multiple languages.
Events
Automatic reactions to things happening in your server, without anyone typing a command: a member joining, a message being sent, a reaction being added, a ban, an AutoMod action, and many more. Use events for anything your bot should do proactively rather than only in response to a command.
Modules
Ready-made features you turn on and configure, rather than build yourself from scratch, for example Welcome Messages, Tickets, Leveling, Starboard, an Audit Log, or Anti Raid protection. Enabling one from your project's Modules tab, filling in a destination channel or a role, is often all that's needed; some modules also open a full workflow builder so you can customize their exact behavior further. New modules are added over time, your project's Modules tab always shows the full, current list, grouped by category (community, moderation, messages, automation), with a search box to find one by name.
Tip: A module's card shows an amber notice if it's enabled but still missing something it needs, like a destination channel, that's the fastest way to tell why an enabled module isn't doing anything yet.