User is a Discord account, the same in every server. You get one as the user of a command, as message.author, as member.user and as the user of the memberLeave event. For what is specific to one server (nickname, roles, permissions), see GuildMember.
Properties
Only known for the bot itself. See Locale.
The public badges of the account, by name. See UserFlagName.
Helpers
These read what the bot remembers: they never send a request.avatarURL()
bannerURL()
toString()
<@id>: mentions the user when put in a message.
toJSON()
Reading from Discord
fetch()
Actions
createDM()
send()
Types
UserFlagName
The name of a public badge, asflags lists them: one of the keys of UserFlags.
Discord documentation
ImageOptions is in Helpers.