Office 365 is Microsoft's new home for SharePoint, bringing the collaborative work environment into the cloud. It joins Yammer, Lync and OneDrive for business — along with the more familiar Office applications — to create a seamless work environment for businesses. Microsoft’s primary message is that Office 365 lowers costs for infrastructure and the people who maintain it. You won't have to create server farms for your SharePoint/Office 365 installations, and Microsoft ensures "five nines" uptime. Because the solution is cloud-based, users can access their workspace from any device, at any time of day. Microsoft can deliver updates more quickly, and has said improvements to Office 365 will outpace those delivered in the 2015 SharePoint Server release.
For some, though, Office 365 is not the answer. Many companies face regulations that require their data be stored in specific locations, or want to retain control over software updates. Maybe you want to decide when updates should be rolled out, and who should get them. By installing the updates yourself you can make sure that nothing breaks. Or you want to continue to use the customizations you've created for your on-premises implementations that are not compatible with Office 365. And finally, you may not be willing to put your critical business data into the cloud, where its protection for the most part is out of your hands.