Channels are a tool for broadcasting your messages to large audiences. They can have an unlimited number of subscribers, they can be public with a permanent URL and each post in a channel has its own view counter.
Technically, they are represented by channel constructors.
Supergroups are a powerful tool for building communities and can support up to 200,000 members each.
Technically, supergroups are actually channels: they are represented by channel constructors, with the megagroup flag set to true.
Gigagroups are something inbetween a channel and a supergroup.
An admin, when prompted by the API using suggestions », can convert a megagroup into a gigagroup using channels.convertToGigagroup (one way only).
After that, only admins will be able to write in the group (like when send_messages rights are disabled for all group participants by default), but the participant limit is removed and the group can become much bigger than a supergroup (e.g. >200,000 currently).
Also, one can't invite people into gigagroups and participants of voice chats in gigagroups are muted by default.
Channels and supergroups can be created using the channels.createChannel method, by setting the appropriate broadcast or megagroup flags.
Supergroups can also be assigned a geo_point to become geochats.
In previous versions of telegram, only normal groups (represented by chat constructors) could be created using messages.createChat: these groups have fewer features, and can only have 200 members at max.
To upgrade a legacy group to a supergroup, messages.migrateChat can be used.
The chats field of the result will have two objects:
migrated_to field, indicating the address of the new supergroupWhen getting full info about the migrated channel, the channelFull object will have migrated_from_chat_id and migrated_from_max_id fields indicating the original ID of the chat, and the message ID in the original chat at which the group was migrated.
All users of the chat will receive an updateNewMessage from the old chat with a messageService containing a messageActionChatMigrateTo constructor.
All new messages have to be sent to the new supergroup.
When working with migrated groups clients need to handle loading of the message history (as well as search results et cetera) from both the legacy group and the new supergroup. This is done by merging the two messages lists (requested with different Peer values) client side.
Channels, legacy groups and supergroups allow setting granular permissions both for admins and specific users; channels, supergroups and legacy groups also allow setting global granular permissions for users.
For more info on how to set and modify rights, see here ».
Telegram allows pinning multiple messages on top in a chat, group, supergroup or channel.
See here » for more info on pinning and unpinning messages.
Groups can be associated to a channel as a discussion group, to allow users to discuss about posts.
For more info on how to set a discussion group in channel, see here »
Both supergroups and channels offer a so-called admin log, a log of recent relevant supergroup and channel actions, like the modification of group/channel settings or information on behalf of an admin, user kicks and bans, and more.
See here » for more info.