A team is a group of people gathered to get something big done in your organization. Sometimes it's your whole organization. Teams are made up of channels, which are the conversations you have with your teammates. Each channel is dedicated to specific topic, department, or project. 

For example, the PGIP Technology (IT Support) team has General, Meetings, Projects and Support Tickets. All the conversations, meetings, files, and apps have to do with IT Support, and everything in this channel is visible to everyone that is added to this team. 

Channels are where the work actually gets done where text, audio, and video conversations open to the whole team happen, where files are shared, and where apps are added. While channel conversations are public to the ones in the team, chats are just between you and someone else (or a group of people). Think of them like instant messages like when using other messaging apps.