This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
Generally looks like a slightly overweight floppy. All current ones are single-sided. First generation 3.5 MO was 128 MB on a cartridge. Second generation devices (available now) are 230 MB. Third generation (due out this year?) will be 650 MB. If they bring the drive price down, could displace floppies as the basic shirtpocket-transportable medium. (rdv,95/1/20)