Realms are Mojang-hosted worlds: a subscription, an invite list, and a backup. A small SMP you host yourself is the same social object with worse uptime. The mechanics of joining either are on the multiplayer page. This page is what you should decide before the first night.
Write these down
- Difficulty, and whether keep-inventory is on. Changing it later is a vote, not a surprise.
- Whether PvP is on. Default should be off unless everyone came to fight.
- How close to spawn people may build, mine or dig a hole to bedrock.
- Whether the Nether roof is allowed. On Java it is a highway. On a small world it is also where unattended farms load forever.
- Where withers, the Warden and TNT are banned. “Not near spawn” is a complete rule. “Use common sense” is not.
- Who is an operator, and that operator commands are not a toy. One person holds the key. A second person knows the backup password.
- What happens if someone wants to leave and take a build with them. Usually: the blocks stay, the player goes.
Backups
Realms keep a backup you can roll back to. Learn where that button is before you need it. If you host the world yourself, copy the world folder on a schedule that is not “when I remember”. The player who was “just testing TNT” will one day be someone in the group, and the only apology that works is a restore.
Invites
A Realm invite is a list you control. Do not post the address, if you have one, in a public Discord. Younger players belong on an invite list, which is also the point of the age and accounts page. The behaviour you expect once they are in is the player rules — copy the ones you mean, and delete the ones you do not.