they're just a connection method, if you search on google you'll get plenty of good explanations. DVI is video only, and digital RGB only. HDMI can carry audio as well, and also has the option of YCbCr in addition to RGB.
there are technical differences that may arise between RGB and YCbCr in some source devices.
You can think of DVI as sort of being a subset of HDMI.
In any case, they are merely connection methods, so don't buy a display purely on this, as it has little to do with the display's image quality.