Color space defines what color is. It defines what color red is. There are three main color spaces for photography sRGB, Adobe RGB, and ProPhoto RGB.

sRGB is the smallest, but most commonly used color spaces. This is the color space you must use on the web since web browsers assume that is what the image is in.

AdobeRGB has a greater range of colors. Many people use this color space when they are working with photos in Photoshop.

ProPhoto RGB is the biggest space of the three. Most people think this is overkill since very few printers or monitors can print the colors that are exclusive to this space.

I did a test one time using my old Epson Stylus Photo 1280 and could not tell a difference between sRGB and AdobeRGB. I now have a Epson Stylus R1800 which has a greater range of colors so I'm going to have to repeat that test. That is what I recommend you do.

If you have an image in either ProPhoto RGB and AdobeRGB and you want to show the picture on the web in Photoshop you need to go to Edit -> Convert to Profile and pick sRGB.