Outlook is Microsoft's well known and highly popular product best known for its email client as part of the Microsoft Office suite. Outlook goes beyond just email, incorporating task management, calendar, contacts and more. It can be used on its own or with Microsoft's enterprise products such as SharePoint or Exchange.
For email, one of Outlooks strongest points is its cross-platform reach. Not only can you get it on the Mac as well as Windows, but there are mobile apps for Android, iOS and Microsoft's own Windows Phone operating system. The recent acquisition of Accompli, a well respected mobile email app, has given Microsoft a quick turnaround on getting a quality product onto both Android and iOS.
Outlook, like many other Microsoft products, is getting a bit of a makeover for Windows 10. On the mobile front there are completely redesigned mail and calendar apps, also available on PCs and tablets as Universal Apps.