Players, or users are characters on ROBLOX with a real person in control. This can be anyone who participates in any game, group, etc. (Not a guest.) Players take part in numerous activities in the community, including buying and selling items, earning player points, editing their avatar, and using the forums.
There are two kinds of players. A permanent account, which, when you log off or leave a server, your player will still exist as well as the data on games (unlike a guest), unless the player is banned. Permanent accounts can be created through the sign up page and accessed by signing in on the log in area. These accounts may interact with the community depending on the chat settings set and safety measures in effect on the account (usually for people who are below 13 years of age). These accounts can also be banned by the ROBLOX Moderators and Administration staff as punishment for breaking rules on the player's account.
Guests, on the other hand, are players who do not have an account, an individual name (usually something like "Guest 1337"), and are present only inside a game server. Once outside of the server, the guest does not exist and cannot interact with the community. This is the only option for playing on servers while not logged in, or without a permanent account. Guests cannot communicate with other players since the removal of Safe Chat, or without the use of certain scripts or exploits. Guests are also the only "players" which have a space on the username (excluding old accounts).