Sharepoint Hub Sites

Hub Sites bring the Communication and Team sites together and become the centralised building block of an Intranet. By creating a Hub Site you can link all of the relevant Team & Communication Sites together making them easier to find, roll-up content from each of the sites (as you can see on the image above the news has and activity has rolled up from multiple sites) to the homepage, provide a standardised look & feel, consistent navigation and simplify access on mobile devices.
As you can see in the Hub Site Screenshot and the Communication / Team Site. We always have the same navigation on all sites as you browse between the sites. This makes the user experience much easier. This was possible previously but required much more manual steps so will save a lot of time going forward.

Its possible to create multiple Hub Sites in your organisation but not possible to link Team or Communication Sites to more than one hub. Hub sites are rolling out to Office365 tenants now and should be available for your organisation soon.
Communication Sites

Communication Sites are sites that are used for sharing information such as News, Events, Reporting Data & Showcasing Key Information such as Products, Teams or Events.