A Team is a workspace that you (or your Teams Admin) can create so that multiple people from different departments can share information. Each Team can have multiple channels within it, helping to focus conversations between specific participants and can display different files and tabs such as PowerBI reports or embedded Excel sheets.
A chat is a relatively ‘flat’ space that primarily allows two or more people to quickly share ideas or files but is not designed to provide as flexible or manageable a workspace as a Team.