Trading Cards are part of a system called 'Community Items'; cards that drop based on play-time, allowing you to craft badges and unlock new wallpapers. They're a 'by invitation' kinda thing, so the game has to sell enough before it gets invited.
They'll drop automatically every 45 minutes of play-time, for a total of 4 cards (after that I guess you're supposed to trade, thus the name). In the case of Wings of Empire, there are 8 new cards, 10 unlockable backgrounds, 5 emotes and a crafting badge.
In addition I made a little 'steam screen booting video'; it doesn't drop automatically, just something to use your steam points on. Devs don't really get anything out of this, beyond a bit of marketing, so it's just something I do for the love of the game.
On the main game side I've already started work on Act+, the sandbox expansion work. Perhaps unsurprisingly Jade immediately volunteered to be the first event. A little cryptic, but it's the only image in the series I can show without tagging the post as NSFW :)
Trading cards should already have started dropping; hope you like them!