I don't know about you, but I find I usually want to do some editing on my photos before printing them or emailing them to friends or uploading them to a photo site. It may be nothing more than slight cropping to fit the output format. Or I might need to straighten the horizon because I tilted the camera when I shot the picture. I might want to adjust the exposure or contrast to improve the photo, or tweak the color balance.
There are many tools available to do all of these. I have my own assortment on my system, and you should, too. You almost certainly got a photo editor with your camera. In addition, there are free tools, and inexpensive tools, and professional tools available. I'll describe mine a bit.
My main photo editing tool is Adobe Photoshop Lightroom. I use it in some manner for every photo I take. Not that there aren't other tools that I could replace it with - it's just that Lightroom is the tool I have chosen as my central workhorse. I use it to import my photos. I add captions and keywords with it. I adjust color, saturation, and use it to print my photos. It is my main program used for organizing and locating photos.
In addition to Photoshop Lightroom, I use ACDSee Pro ($130). It is useful as a quick view of photos in the file structure of my computer. I use it most often to view the photos I have exported from Lightroom. I also use it to quickly email a few photos to friends and family. ACDSee (not the Pro version) is available for $50.
I use Picasa (Free!) for its capability to upload photos to Picasaweb, part of Google. That's where I upload most of my photos to for sharing with friends and family. The latest version of Picasa (version 3) has some pretty decent simple editing tools, and can also keep a local folder on your computer synchronized with a Photo Album on the web. And, it's all free!
There are occasions when an industrial strength photo editor is required. I use Adobe Photoshop (about $600) for that. You may find that you don't need this much power, and that one or more of the tools listed above will meet your needs. Or, Photoshop Elements (about $95), Photoshop's baby brother (with amazing capability for the price) or one of many other less expensive photo editors, will do what you need.
Two of the photo editors I mentioned above perform nondesctructive editing. That is, they do not modify your original photo. Adobe Photoshop Lightroom and Picasa (version 3) fall into this category. Instead of modifying your original photo, they generate a set of instructions that describe the changes you wish to make, and store these instructions with the photo or in a separate database or file. The advantage of this is that your original file is kept intact. If you find later that you want to undo the edits because, for example, you cropped something out of the photo that you wanted to keep, just open the photo in your editor, and undo the crop command.
If your photo editor does not do Nondestructive editing, be sure you never save your edited photo over your original. I personally add a suffix, like "-1", "-2", etc., to my edited photo so that my original is kept intact. For instance, if my original photo is "John at Lake Wenatchee.jpg", after I have edited it I might save the resulting photo as "John at Lake Wenatchee-1.jpg". (Note that this name would not fit my photo naming scheme, as described elsewhere.)