The xD Picture card format was developed by Olympus and Fujifilm but has never really caught on and there are signs that it's now on the way out. When it came out, it was the smallest memory card format available, but today's MicroSD cards are smaller still, so the xD format has no real advantages any more.
For a while, Fujifilm cameras came with combined xD/SD memory card slots so that owners could use both types, but the latest cameras use SD cards only. Olympus, meanwhile, still uses xD card slots in its compacts but now provides a MicroSD adaptor so that owners can use MicroSD cards instead.
