A DVI-I port should be able to carry Digital AND/OR Analog DATA(works with DVI -> VGA Adapters).
Dual link is needed for resolutions above 1920 x 1200.
DVI-D is a digital only version and what is on the monitor end most of the time. It can also be on the video card side depending on the card(many new cards have 1 DVI-I and one DVI-D).
DVI-D cables plug fine into a DVI-I port, but DVI-I cables do NOT plug into a DVI-D only port(The analog pins have no holes on the DVI-D port).
For more info check out
Digital Visual Interface @ Wikipedia.org