Offline accounts are Minecraft accounts that originate from offline1) clients, or cracked/pirated versions of Minecraft. These accounts do not authenticate with Mojang or Microsoft which allows them to assume any name, skin, and owner unless some precautions are taken to prevent the latter from happening. Because of their offline nature, they can only connect to LAN servers or Minecraft servers on the internet that are in offline mode. Offline mode servers usually indicate that they allow cracked accounts, but some are subtle like the cunny.zone Minecraft server.
Offline accounts can benefit users who cannot afford to purchase a legitimate copy of Minecraft or choose to not sign up for a Microsoft account due to privacy concerns. Because of these benefits, these kinds of accounts are frequently used by roleplayers especially those who play multiple characters and typically abandon their accounts frequently after their character dies or is no longer being played.
Issues with the use of these accounts can arise, especially if they are used online and not authenticated through third-party solutions. Because they don't authenticate with Mojang or Microsoft, they natively do not have a unique, user-customizable skin that persists across servers and offline, or any capes. Offline accounts on servers that do not have authentication mods installed can easily be hijacked by another user joining the same server with the same username, which can cause issues with people who want to use their offline account alone.
Mods like SimpleLogin can generate or let the user choose a password on first startup which will prevent an offline account that joins a server from being stolen by another user with the same username on the same server. Authentication this way is served on a first-come-first-serve basis, with the first person to claim a username having sole access to it unless it is revoked by a server admin. This password is stored as an unencrypted file on the user's client and as an encrypted file on the offline server they wish to connect to. When a player joins a server, their password is automatically sent to the server. If their password matches the one stored in the server that was created on their first join, they will be allowed to join and use the account on the server, but if it does not match, they cannot join the server unless they use an account name that has not been claimed on that server already. If someone has already claimed an offline account on a server and someone with the same account name tries to connect with a different password, they will not be allowed to connect unless their password matches the password stored on the server.
Offline accounts can be customized using server-side or client-side mods that can change the skin or other parts of a user. Notable skin mods include Everlasting Skins, which allows offline account users to change their skin to any properly-formatted image acquired from the internet via a URL. Customization mods used in tandem with authentication mods can make them nearly on-par with online (or legit) accounts features-wise, although skin mods and authentication mods can vary per server which can make having a consistent appearance and presence across multiple Minecraft servers difficult as the user would need to change their skin on each server individually if they would like to have a skin or update their skin over time.
Offline account users have a reputation of being degenerates, cheapskates, or anything else typically undesirable. Although most offline account users are not griefers, hackers, laggers, or terrible people, the low barrier of entry inherent to offline accounts allows for many people to cause problems in Minecraft servers.