The Royal Flush Gang are a group of thieves who get named based on the playing card they're attached with. More times than not, it's a family of thieves working together to ensure not only success, but a less chance of betrayal.
Though they came to be in the comics, they really took off in the cartoons via Justice League and Batman Beyond. In the Justice League cartoon Joker was the one who put them together. While Batman Beyond had them be a legacy of theives that was the family I mentioned earlier. Their gear usually includes flying playing cards that they travel around with, along with various other tools and gear to get the job done.