Communication Site vs Team Site

Hi all,  I'd like to add to the discussion from my experience. 

I believe that the best way to explain the difference between a Communication Site and a Team Site is in how they are meant to be used. Team Sites have an O365 group associated with them and, therefore, can be a great resource for team collaboration (e.g., executives working on a project together who need to be able to share notes, access documents, etc). For this purpose, a Team Site will include a distro list, a team calendar, a Planner, a OneNote, etc. This Team Site can be public or private and the O365 Group associated with it will be displayed in your Outlook (under the "Groups" section of your panel) allowing members to easily access files in the site, email all other members, view the team calendar, etc. Additionally, when you create a Team Site, you can attach the site to an MS Team to take advantage of the features offered by MS Teams (persistent chat, call conferencing, connectors, etc).

Communication Sites, on the other hand, are a great way to create and promote compelling information with others (e.g., we use Communication Sites in our adoption campaigns, e.g., Office Adoption Campaign) but, because Communication Sites do not come with an O365 Group, you are more limited in what you can do with them (i.e., you cannot attach a Communcation Site to an MS Teams for instance).

Beyond these differences, you can make a Team Site look the same as a Communication Site as the webparts and navigation are the same. This makes sense as you may want to have a nice looking Team Site to share information with your team.

I hope this helps.