difference of channels and groups
info from Rocket.chat Documentation:
Public channels (also called channels for short) are the most important kind. Use channels for conversations that are open to your entire team. A channel can be joined by anyone on your team.
Private groups are closed channels. They are visible only to their members. Use private groups for subjects that are sensitive, confidential, or limited to a small group of team members.
info from Rocket.Chat repository:
Channel: 0 to n person. All messages need to keep for ever even if all people exists.
Group: at least 2 to n person. remove the group and messages if all people are exists.