Outlook is a free internet-based email service and software program provided by Microsoft. It is an application you can use to send and receive emails. It is kind of like Google Gmail Service, but it is more upgraded as it links to your desktop outlook data. 

Microsoft Outlook started in 1997 which was included with Microsoft Office 97. Then Outlook Express was included with Windows XP, which was the only free version. Since then, Microsoft has released a number of updated versions, each with more features than the previous one. Outlook comes with many of the Microsoft Office application suites, including Office 2010, 2013, and 2016, and Microsoft 365. One important point is that not all versions include Outlook. For instance, Microsoft Outlook is not included in Office Home & Student 2016. But is available in Microsoft 365 Home.