Joining servers

When you cannot join, and what online mode and offline mode mean.

Can't join a server?What to do when a server is sleeping or offline, asks for a resource pack, runs another version, checks Minecraft accounts or has a whitelist.
Offline-mode servers and sign-in pluginsWhat it means when a server's names are not verified, how a sign-in plugin protects players, and what does not work in offline mode.