Memories will display old snaps that you captured in certain places, with the idea that it'll help you relive your favorite moments. Users will only be able to see their own memories in this Layer, which sort of builds on the My Places feature Snapchat this summer. As for Explore, that's a reimagined version of the heat map. You'll be able to check out places though photos and videos that other users have shared publicly. You can access the Map Layers through a menu at the top right of the Snap Map.
is also opening up Layers to third parties. A Ticketmaster layer showing nearby concerts (and providing a way to buy tickets) is coming soon, as is one from that highlights the best restaurants on the map. More Layers are in the pipeline too.