Steam Trading Cards are a cute little bonus you can get for your spent time playing games in your Steam Library. It's quite simple: play your games and you'll unlock most of the Steam Trading Cards available in an eligible games. However, maybe you don't have the time available to play everything you've got in your Steam Library and collect all those cards. Idle Master may be your answer.
The idea behind Idle Master seems fairly altruistic in it's nature as written on.
"Do you have a lot of games in your Steam library that have card drops available, but not enough time to play them all? How great would it be to run a program before bed and wake up to an inventory full of Trading Cards? That's what Idle Master does."
Basically, Idle Master is an open-source program that's available for free as a "Labor of Love". The program is meant to be set up and left, as its name suggests, idle while it scans through your Steam Library and actively tricks Steam into thinking you're playing any games eligible for Steam Trading Card drops. Sure, on some level it might not be so honorable, but it's also not as though Steam's going to go out of business on people finding an easy way to access their free card collectibles.
So if you dig that kind of thing and want to get yourself some digital goodies, but don't have time to spare on everything in your library, consider Idle Master.