"We want to let you know that in the coming months, we're upgrading the content you created in My Maps. All of the maps that you've created will automatically move to the new Google My Maps (previously called Google Maps Engine). Classic My Maps on maps.google.com will no longer be available."
So all the custom maps you've created in the old Google Maps interface will be automatically migrated to the new My Maps. If you don't want to wait, you can go to My Maps and click "upgrade now".
"Upgrading your maps is permanent and takes just a minute or two," informs Google. "The new My Maps is a more powerful maps creation tool that makes it easier to edit and share maps you've made. Just like with classic My Maps, you can work on a map with others, save driving directions, add photos and videos, and more." There are some limitations and restrictions, since Google also has a paid version called My Maps Pro, which is better suited for big maps.
The new My Maps (previously known as Maps Engine Lite) is more powerful than the old My Maps as it uses Google Drive sharing, has support for layers, it lets you import locations from spreadsheets and more. The main difference is that My Maps is a standalone site and you can't create custom maps inside Google Maps. You can see a list of recent custom maps and there's a button that lets you create maps.
Google has an Android app for My Maps and a gallery that features publicly shared custom maps. "The possibilities are pretty much endless—students have photographed and mapped a city's public art installments, authors have laid out their stories' locations on the map, and activists have plotted out shelters and distribution centers during emergency situations," mentions Google's Lat Long blog.