At the moment these include: San Francisco, Peninsula & San Jose, Northern California, West Los Angeles, Manhattan, Chicago, Boston, Washington, DC and the Midwest. The stores available differ by location. See here.
As it turned out I was in Downtown San Francisco so the service was available to me and they really punted it out as I was searching on Google.
It’s free delivery for a certain value of goods when you buy from the various stores they deliver for: for example, I spotted Target and Costco linking to the system. Then you choose a delivery window and get the stuff you want. Orders over a certain value are free but there is a subscription service too.
It’s an interesting new horizon for Google that, as far as I can see, they are only testing in the States just now but it’s notable to see Google make a foray into delivery.