The postcards are digitally printed on A6 400gsm card, matt coated on the picture side. They're posted once a month from the printers in Axminster, Devon in the UK.
The back contains information extracted from the time, date and place the photo was taken. It also currently includes any caption you might have written.
The exact format is likely to change over time, in response to user feedback and generally getting a sense of what works after living with the cards for a while.
You can see examples of what the cards currently look like and how they've changed over time on the Pastcards Instagram account.