The MiniDV (Mini Digital Video) is the high-end consumer (or ProSumer, whatever you prefer) format of digital video tapes.
MiniDV is the little brother to DV tape systems, and is in fact based on DV when it comes to specifications. It was supposed to be a simpler version of DV, for consumer use.
However, unexpectedly, the MiniDV format became very popular among high-end users, and many semi-professional and advanced amateurs choose miniDV over other formats.
The dimensions of the miniDV tapes are approx. 47 mm x 66 mm x 12 mm. The tape inside the cartridge is 6mm wide.
The tape inside the cartridge is completely encased when the cassette is closed. Upon inserting the cassette into a camera or player, the tape itself is exposed (think VHS cassette, but very much smaller).
The MiniDV format supports play times of 30, 60 and 80 minutes.
Some of the more high-end MiniDV camcorders support an IC (integrated circuit) chip built into the miniDV tape. This IC chip can store information about shutter times, apertures etc. To store this information, the camera needs more expensive tapes marked with "IC". Non-IC cameras can use IC tapes (but can, obviously, not store anything to the extra chip), and IC cameras can use non-IC tapes.
Also see:
MiniDV vs. Digital8
Other video formats:
Analogue: VHS - 8mm - VHS-C - Hi8
Digital: Digital 8 (or D8) - DV - DVPRO - DVCAM