Hello,

Communities allows for various users to have access to various information. You can think of it as a portal that has Chatter.
For each community you can customize the tabs, profiles/users having access, logo, add a little bit of personalization when it comes to colors.

The most important is to define which users will have access to which information.

When making this choice, think of the following:
- What will distinguish each community
=> will a community user possibly have access to multiple communities? this can become a problem with chatter feeds (mentioned later)
=> will internal users have access to multiple communities (to manage them)?
=> if so, you might have to consider creating some training material for all those users

- Chatter groups can be accessed within their own communities only, and not visible from the main org
=> is some of the content expected to be available to multiple groups of users that were considered to belong to different communities? If so, content management will be an issue

It seems to me that communities that are profile-driven make sense (ex: one community for customer service, another one for warehouse management with partners etc), rather than 'data-driven' (which Product did this user buy, and adding the user to a specific community as a result). This is my personal opinion however.

Also think of the time it will take for internal users to navigate all these communities, especially if the plan is to have them update a lot of content. It could result in a little bit of frustration.

There is obviously more to it, but this is what I can think of for now, and hopefully you can extract the pros and cons in the above points.

Nathalie