a FireWire port - also known as DV port... or IEEE1394 or i.Link. The camcorder manufacturers (JVC, Panasonic, Sony, Canon, Samsung and the rest) never put a FireWire cable in the box, so you need buy one... 4-pin-camera... 6-pin-computer. Don't go cheap. I use a Monster FireWire cable... 6 footer was $25. Gold contacts, works great.
According to the GR-D650 manual
See page 9, under "Connectors" - The DV port is near the USB port... Item 21.
Also note on Page 12 - no i.Link cable.
See page 44... Note the "(optional) DV cable"...
From what I can tell, this JVC is a bottom loader. If you use a tripod (which you should), this makes changing tapes a bit of a challenge. I use a Sony HDR-HC1 and have learned some "work-arounds"... but they are not efficient. My son uses a Canon Elura 60 on his G4 PowerBook - take a look at the Elura 100. It is a top-loader... and 20x optical zoom compared to the JVC's 15x...
Other tips: Use a tripod. Turn OFF digital zoom - use only optical zoom. Have fun.
Buying my first camcorder, for use with a Powerbook G4 (which will probably be replaced with an iMac with Leopard next year). I don't plan on buying anything fancy and hope to keep it to around $300-$400. Is there any camcorder in that price range that seems to work better than others with Macs? Or, is there any reason to think the JVC GR-D650, which seems to suit my needs, wouldn't work?