Meagapixels—Millions of pixels describes camera resolution. And yes, they tout the number of meagapixels to confuse you. High is good but when looking for a camera try to find the number of effective meagapixels. Effective megapixels are the number of pixels used to actually capture the image. A nice point and shoot over 4/6 megapixels will do you fine. A 4/6 will do fine but a little higher will produce a nice image for the occasional enlargement.
PPI—Pixels per inch refers to image file and monitor resolution. I usually recommend web image files be saved at 72 DPI ( I know, I know—just know the 72 part). However, monitors have higher resolutions so 72 is fine but if you want a really sharp image try 85 or 125. If you use a higher resolution be mindful of the size. Large files can still take a long time to load. As far as image file size goes the higher PPI means the image contains more information and can be made larger and still look crisp.
DPI—Dots per inch. This means how many dots of ink will be placed on the paper by a printer. The printer being inkjet, laser or commercial. Higher isn’t always better here either. You can have an image that is 1200 DPI but if your printer can only print 600 or more often 300 DPI it will resample (rewrite) your file and discard the extra information. You and your imaging software are much better at determining what stays and what goes in your image.
FYI: Newspapers are printed using LPI or lines per inch and generally are printed at 85/100 LPI. Ay yi yi.