Those are presets, and the difference has to do with your frame size (NTSC DV is 720×480, for example, and NTSC D1 is 720×486), pixel aspect ratio, and frame rate. Basically all of the different attributes of a composition that you can set. The presets just give you different standard compositions that are already set up for you.
The format that will go on a website is going to vary depending on what sort of player you’re embedding in. YouTube, for example, will accept a pretty wide variety of file types and sizes. Same with Vimeo. If you’re embedding it into your own custom player for your own website, then you just need to go with whatever specs work for that.
A DVD, on the other hand, is pretty specific. For that you’re going to drop down to standard definition for sure, among other things. But since DVD isn’t your only deliverable, what’s best will most likely be to create everything in After Effects at the highest quality level, and then convert your finished file to DVD format later on using Media Encoder or Compressor or whatever program you’re more familiar with.
Without knowing more about your specific situation, I wouldn’t want to make too strong of a recommendation, but if you’re just looking for an extremely general suggestion, I would say to create your composition with these settings:
Width: 1920
Height: 1080
Pixel aspect ratio: Square pixels
Frame rate: 23.976
Again, specific situations call for specific composition settings, but this is my standard setup that I use if the choice is up to me.