To elaborate on what Steve said:
a) If your camera is capable of this, ALWAYS store your pictures in RAW format. This will give you much more (and better) control over colors and tones than trying to do the same on a JPG. If you can do this, your first step should ALWAYS be to use ACR to interpret the raw data. If you can't store your pix in raw form and are limited to only JPGs, it's still a good idea for your first step to be ACR, but in this case, some of the tools within PS (eg. like "curves") can be made to work equally well.
b) "Channels" is not just another tool, but it is the fundamental concept that states that a photo can be broken down into either 3 (eg, R, G, and B channels) or 4 (eg, C, M, Y, and K channels). Associated with this decomposition, there are several tools and panels available to display, modify and even use to modify the 3 or 4 channels in an image once take into PS.
HTH,
Tom M