What is the difference between a Group and a Channel? Can a member comment in a channel or it is just the owner who can post?
channel: only creator and admins with the "can send post" permissions can send posts and members are only viewers of the messages.
groups: as default all the members can participate in the groups but in the new version of telegram, admins/creator can restrict other users and they may not be able to send messages.
From the Telegram FAQ:
Telegram groups are ideal for sharing stuff with friends and family or collaboration in small teams, they can have up to 200 members and by default everyone can add new people and edit the name and group photo.
Channels are a tool for broadcasting public messages to large audiences. In fact, a channel can have an unlimited number of members. When you post in a channel, the message is signed with the channel's name and photo and not your own.
Read more about channels in the Channels FAQ.