A quick follow-up to my post a few weeks ago on dispelling the 72dpi myth. In that post I mostly focused on the intricacies of resolution and why 72dpi doesn't really make sense. The one thing I left off from that post is when do you need to worry about resolution? Or more accurately, when do you re-sample? One of the comments from my post on the 72dpi myth talked a lot about images being resolution independent. As long as you're not changing the pixel dimensions (either adding or discarding pixels) you can freely change the resolution of the image, with no degradation of quality (or change to the file size). The point is that it's all about dimensions. When you're working on screen, you only have pixel dimensions to work with. When you're printing, then you can start thinking in terms of inches (or centimetres if you work in metric). The resolution only becomes an issue then when you're printing.
I think most of the confusion around resolution comes from the idea of re-sampling. Throw that into the mix and suddenly things seem to get really confusing when they shouldn't. Re-sampling really only does 1 of 2 things, it either makes up pixels or throws them away. So when do you use it?
Making something bigger
Sometimes you may need to increase the number of pixels for an image. Maybe you're trying to scale up the image on screen larger than the original. Or maybe you're going to print a very large print and have determined that the resolution will be too low if you don't scale things up. In both of these cases, re-sampling is your option. There are other tools available for this but they essentially all do the same thing, they increase the image to a target size while maintaining a target resolution.
Making something smaller
These days probably the much more common case will be that you want to make an image smaller. If you're shooting full-res images with your camera you almost certainly want to shrink them down before posting them on-line. For making things smaller we're almost always talking about pixels, you rarely have to worry about an image being too high resolution for printing, unless you're sending the files to a lab and they have a file size restriction.
Save your master files!
That's really about all there is too it, don't worry about re-sampling unless you're making something bigger or smaller. Even then, simply worry about what is important to you. If you're reducing the size for the web then it's only the pixel dimensions you'll have to worry about. The last thing is though, make sure to save a copy of your file in its original resolution before you re-sample. Whenever you re-sample you're going to degrade the quality so you're going to want to make sure to keep your master files untouched when you do this. Make sure that you're saving a copy of the re-sized file.
Lightroom makes all of this easier
Adobe's Lightroom does make this a lot easier, you're not touching your original files for starters. Plus, you only deal with resolution when you export or when you print. If you're exporting for the web, just set the pixel dimensions you want. If you're exporting files to send to a lab, then just set your physical dimensions and your desired resolution. In both cases, Lightroom will do the appropriate re-sampling if necessary during the export process. I don't have any experience with Apple's Aperture but I would imagine that things work similarly for it.