The Eye-Fi SD card looks and feels like a regular SD card but within, it's got an 802.11b WiFi interface. It can send images or videos from a camera to a PC or an online hosting service, thus eliminating the need to carry along your notebook PC, USB cables and other dongles.
On the inside of an Eye-Fi card is Samsung Flash chip and an Atheros WiFi chip. You can see a pretty details snapshot of it on IkonTools:
In a recent incident, a stolen camera had a picture of the felon sent via WiFi along with other vacation pictures. You can read more about the incident here: