Channels
Inside of a Team, you have channels. Each channel can be devoted to a particular topic. You can have conversations with anyone associated with that channel, have video meetings or calls, upload files, use apps or calendars. Think of your channels as conference rooms where you can sit down and brainstorm or get work done with your co-workers or peers. Only members of that Team can be a part of these Channel discussions.
Chats
Are not associated with a particular Team. They are less formal ways to communicate. Think of chats as the conversation in the hallway or stopping into someone’s office for a quick conversation. Just like an informal chat in the hallway, you can always add someone to a Chat to have them be a part of the conversation. Members of chats do not need to be in the same organization. In Chats, you can have files and video calls, but you won’t have a built-in calendar to share.