Have you ever taken a photo that you really liked, uploaded it to the Internet and noticed that the colours are off a little?
Well, that is because your image's colour space is anything but sRGB, which is what the Internet uses and properly displays.
To make sure that your image is to that specific colour space, press Command+Shift+Option+S on Mac in Photoshop to open up the Save for Web dialog box. There, by default, the Convert to RGB checkbox is ticked. When you save your image, Photoshop will switch the colour space to sRGB for you so that it renders well on all Web browsers. Beware, though, that sRGB is pretty much the smallest colour space and it will compress your colours. You will lose some data so don't Save for Web until you are sure that it's going to be your last retouching of that file.
The Save for Web command is really useful as it does everything to prepare your file for the Web. You can even export GIFs out of there.