ADR Cartridge's?

[hmsnacker123]

Hi, my Parents have recently bought a Caravan and when i was looking around in it, i found a thing called a "ADR Cartridge", it is 30GB and im puzzled as to what it is or what does it do, and i was hoping that one of you's could help. Thank's in advance.
___

Edit: wow, i was doing a lil' research on them and look what i found:
Manu.
Verbatim

Price Each ex. Vat
£23.00

Pack price ex. Vat
£115.00

Pack Price inc. Vat
£135.13

So there pretty expensive, do'ya think i'll be able to sell it on eBay?
__