To answer out front where I think your question is going, an SDIO card can only be used in a device which is SDIO aware, so check your vendor specs or contact their help desk if you wish to use an SDIO wifi or bluetooth card but aren't sure it will work in your device.
The longer answer is that SDIO is a card standard that allows SD cards to not only store memory, but to provide external input/output to your device. Currently it is most commonly used for WiFi and Bluetooth add ons but I have read proposals that discussed using them sort of like miniature PCI cards for external physical connections as well, with USB/Firewire/eSata connectors.
Typically SDIO cards are longer then normal SD cards, so they don't fit into some slots and usually can't be covered or fully internal.
In order to use and SDIO card, the proper drivers need to be installed and the device/card slot needs to be SDIO aware/enabled. Part of the standard for an SDIO card is that they will not damage or interrupt a device that is not SDIO aware, rather the card will just be ignored. SDIO slots are backwards compatible, and can be used as standard SD slots.
Here is a full technical breakdown of the standard from SanDisk.
http://www.sandisk.com/Assets/File/O...IO_specsv1.pdf
Hope that helps,
Z
