Microsoft is using Release Preview Insiders to test some changes for the Microsoft Store in Windows 10. The latest update brings the Microsoft Store on up to version 11806.1001.5.0 and brings some UI changes and more to the experience (via MSPoweruser.)
The biggest change with the update is the introduction of a new "My Collection" side panel which divides up your applications into owned, installed, ready to install, and downloads. You'll also notice that there is now more spacing between items on the home screen, and changes to the "Show All" icon. Finally, there is a fix for the hamburger menu and a problem which caused the Store to crash on Windows 10 Mobile.
This is not the first time that Microsoft has been testing changes for the Microsoft Store on Windows 10. Earlier in the month, Microsoft tested Wish List, remote app installs, and more in the store. As usual, you can expect for this same update to roll out to everyone else soon, so best to keep tuned for more in the weeks ahead.