What is the difference between Outlook via web vs. desktop?

Outlook 365 on the web is available at portal.office365.com . 

Outlook 2016 (or latest version) is installed locally on your computer. Instructions for installing are available here: https://support.itjones.com/hc/en-us/articles/218745578-How-do-you-install-upgrade-Office365-on-your-PC-or-Mac-

Other than where the app can be accessed, the main functions are pretty much the same.