It is actually called Selective (or partial) desaturation
(it is NOT colourizing or colouring - that is a totally different technique used on b&w images).
For Photoshop, Elements and Gimp:
1. Duplicate your background layer.
2. Desaturate that layer (i.e. make it b&w).
3. Apply a layer mask to the layer.
4. Use the black brush to paint on the layer mask (make sure that is where you are, not on the image itself). That will bring the colour back to those areas. If you make a mistake, change the brush to white and go over it. Using a masking technique is best because you can keep adjusting it and fixing mistakes, which can't be done easily with other methods such as the eraser.
5. When you're happy, flatten and save.
If you don't have any image editors, you can do it for free on www.picnik.com in their effects section.
Here is how to do it on picnik.com:
Upload a photo
Click on the "Create" tab
Click on "Black and white"
In the "Effect Painting" box, choose "Original"
Choose a brush size, then simply paint the color back in where you want it.
If you make a mistake, change it to "Effect" and go over the mistake.
Save the picture.
Here is how to do it on Photobucket (see how useful help files can be?)
http://pic.pbsrc.com/help/PhotobucketHelp.htm
Don't bother looking for a camera that takes "colour splash" photos - that function only works some of the time, and it means you don't get to keep your original in its full glory. If you happen to take a truly fantastic photo whilst on that setting, you'd be kicking yourself that you only have a copy with weird colouring instead of a proper one.
P.S.
This question gets asked many times every day.
A quick search on Yahoo Answers would have brought up countless responses.
Look/scroll up right now - see the white box right next to "Search for questions:" - that's the search box.
Also, be aware that this is a terribly overdone technique, usually done in a very tacky way.
However, I do understand that to YOU it is new and exciting, and you need to get that phase out of your system. Enjoy :-)