Why Calibrate Your Monitor?

Why is monitor calibration important anyway? 
Great question.
Glad you asked.
The simple answer is that each sensor, each lens, each monitor, each printer etc. can render colours differently.
Your camera monitor may display jpg images with a colour values at one level, while the computer screen that you review it on may render them at a completely different level. Then when you go to print that beautiful flower image, the printer may reproduce the values different yet again.
Printing and the calibration of printers, is an entirely different conversation. So let's stick with the monitor at the moment.

When a monitor is properly calibrated, it is set up to display all the colours it is able to display. The screen is not too bright, so that the colours are washed out. Not too dark so that they lack richness and saturation. Zooming in to pixel level, the viewer can delineate between colours easily. Colours show their full potential. They show how the image was captured on the sensor (or film) and don't lack for anything.  Accuracy is the name of the game.

If for example you were to photograph a person with the intention of sharing the image, or printing it, it is highly possible that an uncalibrated monitor would display the tones incorrectly. Skin could show up greenish, the bright blue sky could be dull and lifeless. Lacking of saturation and contrast where in reality it existed in abundance.
It also means that retouching time would probably be longer than necessary. Adding more stress to the retoucher (perhaps that is you). Calibration is an important aspect of an effective workflow that starts with the image capture.

How calibration is achieved can be a long discussion. There are tools that are designed specifically for the task. Prices very. The process is rather simple, but takes a little bit of time. No matter. It's absolutely worth it, and once it's done you only have to follow up every two weeks or so (in most cases).
Good quality monitors designed for the graphics and photography markets, will have highly detailed adjustability so that you can fine tune your monitor to suit your work environment and uses.  A quality calibration tool will work with these adjustments in order to make sure your calibration is right on the money.

Some of the more high quality monitors available are from manufacturers such as: Eizo and NEC.
For calibration tools, look at those by X-Rite, Datacolor and others.
Calibrating your monitor is the start to a better workflow and better results. You WILL see the difference.