A standard for that accessory devices that extends the functionality of devices with SD card slots.
While an SD card is a memory card that simply stores files, an SDIO card can be complete hardware accessory. Examples of SDIO cards include Wi-Fi and Bluetooth adapters, GPS receivers, TV tuners, cameras, RFID readers, and fingerprint readers.
SDIO cards are often longer than SD cards, and thus "stick out" from the device into which they are inserted.
A device with an SDIO card slot can also accept standard SD memory cards, but not necessarily vice-versa. A device must specifically be "SDIO-capable" in order to be compatible with SDIO cards.