MIDI Interfaces are the connection between the Computer and musical instruments like synthesizers and drum machines and sequencers. It's a 5 Pin Jack, read more about MIDI Basics - In deutsch MIDI Format
Unfortunately there are not many Manufacturers left, who still produce MIDI Interfaces (most of them USB -> MIDI with 8 ports (means 8 5-Pin-DIN Jacks) in and 8 posts out.
Interfaces - AMT8 + Unitor Series
A very good and well known Interface series was the Emagic AMT8 and the Unitor 8 Mk2 (Mark I had no USB but RS422 instead). Apple discontinued the interfaces along with the great Sounddiver MIDI Sound Editor. Anyway, the AMT Technology and the Interfaces themselves were good and fast. The MT4 was a cheaper version without AMT which wasn't the same level but entry level MIDI for everyone and laptop computers.
- No problem on Macs with multi-client apps, this is a matter of the drivers for Windows XP and Vista. Newer and more modern OS'es got a generic driver for MIDI Interfaces like OS X Tiger and later.
Current MIDI Interface Manufacturers
these are still available and work fine. some may need drivers on Win XP to work as multi client.