1 What is it?

2 What are its features?

The special features of this Olympus card relate to their own brands of camera.
It can allow panoramic shots to be taken, stored and then compiled in conjunction with their own software.
It has also been programmed in such a way that a variety of art functions can be utilised when this card is used with Olympus Master software.
The memory chips used are fast in handling memory both in input and output terms.
A camera user will find this helpful when taking pictures or video on a digital camera, and wanting it to be stored very quickly, so that the next shots can be taken. If you had a really slow memory card, you might miss Granny making a fool of herself at the wedding, because the last picture was still being saved to memory.
The same speed of memory is helpful when downloading the pictures to your PC afterwards. It can be a long enough process downloading a whole batch of pictures, at the best of times. A faster memory will speed up that process.
When you realise that a 7 megapixel camera in high quality mode could squeeze over 1000 pictures on to this card, you can see why faster is better.! If absolute speed is important to you, you should check out H cards, which are faster still.

As with most digital cameras, there is a slot in the camera body into which the card slips quite easily. You simply open the proper 'door', push the card into its place, and start taking pictures.
Some card slots are in awkward places, and access may be partially blocked by batteries or other card slots, but increasingly camera manufacturers are avoiding that annoying process.
I am using mine on the Olympus E-400 that I have reviewed. It did not come as standard, but was purchased online for around £20. That price has not dropped much due to a bit of a memory shortage at the moment.
In this particular camera, it is possible to use either the xd card or a CF ( compact flash ) card, or both together. It is possible to move pictures from one to the other while they are in the camera, but such facility is usually restricted to SLR type cameras, rather than the compacts.
They are well made, and are provided with a 5 year warranty.

They are slightly more expensive than generic cards and also than SD cards of similar capacity.
I find them small and a bit fiddly compared with the CF cards - but that is more a reflection on my eyesight, age and chunky fingers, probably!
There are not a great number of other camera manufacturers who use them, so they might not be interchangeable with all the other camera users in the family. Fuji do use this type of card and do recommend them.
There are a few Olympus cameras which cannot use them, and it is worth checking the Olympus site to be sure.
Other specialist card types in the past, came and went.! I got left with a Fui compact which uses SmartMedia cards. They were all the rage when I purchased the camera, but never got above 128MB and are almost impossible to replace today. It would be very annoying if the XD card suffered a similar fate in a few years time.