Portals were introduced into Dynamics after the purchase of ADX Studio. Portals are hosted by Microsoft and free to use (providing you have less than 1,000,000 views per month). A Dynamics Portal can be used to publish content and to allow portal users to view and update Dynamics data. The administration and management of the portal is done from the Dynamics Admin Centre under the Office 365 Admin Center.
You can select the type of portal and target audience you want when you configure your portal.
Dynamics Portals utilizes the Bootstrap framework to provide portal users with a responsive user experience across devices of all sizes. You can easily customize the Bootstrap Html/CSS to suit your own design requirements and there are plenty of tools and online examples on how to do this. Its also has a template language called Liquid which you can use to add dynamic content. The major frustration for many users is the inability to update CRM from Liquid. The previous version of ADX portal supported this and hopefully it will be introduced soon.
Portals have their own security model and you can configure how users are authenticated and create Web Roles for access to data. The security model also supports record based security. For example it could be configured so that a user only has access to account records where they are the primary contact.
Dynamics 365 Portal are easy to configure and an easy way to quickly create a portal without specialized skills and having to worry about things like hosting and authentication.