I don't intend this to be a be-all end-all post about resolution, but something to give you what you need to understand what it is, determine what resolution you should be working in, and what resolution your final output should be. Resolution questions seem to be common on message boards. So I thought it might be a good subject to attack.
What is Resolution?
To put it simply resolution is how many pixels wide by how many pixels high an image is. This is typically measured by inch, as in PPI (pixels per inch), or as it correlates to the print world as DPI (dots per inch). Since a lot of digital work is done with print in mind, people typically refer to resolutions in DPI no matter what the final output is, and so will I.
Why do we need to even refer to resolutions "per inch" in the first place? Well, if we were only looking at the image on a screen, we wouldn't. Since we like to print stuff, though, we need to know how many pixels correlate to how much space on a page. So an image can be 1000 x 1000 pixels, and then we can choose to give it any DPI, which will effect its size when printed. Which leads us to the all important topic of:
Screen Resolution Vs. Print Resolution
Here's something definite: Screen resolution (the resolution of your computer monitor) is 72dpi. Even if there's a higher resolution assigned to the image, it will still display on screen at 72dpi. That's not saying it shrinks: a 1000 x 1000 pixel image will display on screen at 1000 x 1000 whether it was assigned 72dpi or 1200dpi. It's a good habit, though, always to save things intended for screen at 72dpi.
Less definite is print resolution. Why? Because the optimal DPI for print can depend on what you're printing. Its also dependent upon the capabilities and limitations of the print house. Since we're talking about comic books here, let's look at some good rules of thumb for comic books, and various printed publications (keep in mind these rules may not apply to, say, billboards).
The basic rule for print is 300dpi minimum. This is generally considered the point where there is no pixelation visible to the naked eye. What I'm talking about here is 300dpi at print size. That is, if you wanted to print a 5" x 5" page, at 300dpi, that would be 1500 x 1500 pixels. Get it? Got it? Good!
There are a lot of folks out there who, I'm sure, are ready to flame me for suggesting 300dpi as a minimum. But I can say, in good faith, as a graphic designer working a lot in print, 300dpi is usually all you need for your final files. An exception would be straight black and white linework, which tends to show off some jagged edge pixeliation at 300dpi. There I would suggest shooting for 600dpi if at all possible.
In any case, if you can work/save/print at higher resolutions than these, I say go for it. Operating at a higher resolution leaves options open for other applications. Forethought can save you a lot of trouble. I know a lot of webcomics artists who've saved all their work at a good size for screen display, but are mortified when they find out they can't use those same files to print! Its a good idea to work at a high resolution, and save new files with different names at the smaller sizes as needed. Which leads me to the next point:
You Can't Scale Up!
You can always scale down, but you can never scale up. Well, technically you can, but it will look like crap. Why? Because when you scale up, you're asking your software to invent data that isn't thre. If I have 1" square at 72dpi, thats 72 x 72 pixels. If I scaled it up to a 1" square at 300dpi, it would be 300 x 300 pixels. This means you're asking your software to create over 84,000 new pixels to fill in the gaps. The result is usually a blurry out of focus mess.
Likewise, trying pass off lower DPI images won't work either. The result there will be a pixelated, mosaic look. The moral here is: do it right from the beginning.
Applying This Information
A lot of the detailed applications will depend on the specific software that you're using, and since there's such a wide variety of software out there, it would be impossible for me to cover every single one. So, I'm going to generalize a bit here.
When working completely digital, resolution should be your first consideration. Most programs ask as soon as you create a document, for dimensions and resolution. Pretty straight forward. Exercise some forethought, though, as the resolution you start with is the highest resolution that your piece will ever be. Also take some care not to accidentally save over your higher res versions if you're scaling them down. A good habit of saving them into different folders and having a good naming system can help there.
If you're going to be scanning in artwork, there's a bit more consideration, as typically illustrators work larger than print size. The best advice I can give you here is, again, aim high. You're going to be scaling it down for print, but its not a bad idea to scan your work at 300 or 600 dpi anyway, then use scaled versions to work on/print. You may have use for the larger source files down the line-- seems like no biggie, until you have to re-scan your 500 page epic. If you sell your originals, you're going to want to have a good scan to fall back on as well, since you'll never be able to re-scan it.
As for scaling down, I can't tell you exactly how to do it in your particular software (although in Photoshop, which I hear is a popular program, you would go Image>Image Size to open a dialog box where you make your adjustments), but I can give you a couple pointers. Again its pretty straight forward as long as you know what you want your final output to be. You'll want to constrain the proportions, which means, if you set one dimension, the other dimension automatically assumes the correct proportion. Also make sure that the DPI is correctly set. If you sent out a 1500 x 1500 pixel image set at 72dpi, don't assume that your print house will understand that you MEANT a 5" x 5" at 300dpi. You'll just be setting yourself up for a hold up, lots of back and forth, and probably some extra pre-press charges. Getting it right will save you time, money, and a lot of stress.
Most, or all, print houses will have formatting specs for your files. One of the things that they will spec is a minimum resolution. Make sure that your files meet these specs to ensure getting back a quality print. As digital proofs (ie - PDFs) become the norm, it will often be hard to determine print quality. Having your files meet spec, adds an extra layer of confidence. Often they will have a maximum DPI, too. Remember, I said that 600dpi was optimal for black and white linework? It is, but some places print at 300dpi, period. In which case, you're stuck with having to go 300dpi (another time we'll go over some tricks for making that look better).
Hopefully that's not too much info to begin with. The post turned out MUCH longer than I had expected! And, believe it or not, that's the tip of the iceberg as far as what we could discuss. This will probably be a good post to refer back to, also, when we're discussing other matters, and it really is one of the most common questions that I see come up on message boards.