Why is knowing about DPI and PPI important? Because the companies that sell digitizing equipment use higher resolution as a sales factor in introducing new products. For example with digital cameras, if you have been aware over the past few years, you will be aware of the Megapixel race. The first digital cameras had image sensors of one or two Megapixels, now the cheap cameras claim 10 Megapixels or more. The new Canon 60D is an 18 Megapixel camera. The key here is not the Megapixel count but the quality of the lenses and the rest of the camera mechanism. No matter how much resolution your camera's sensor has, if the lens is lousy, the picture will be lousy. Since it is now the holiday season (2010) you may be thinking of buying into one of these devices, such as a digital camera or a scanner, so you might as well have some perspective before you buy.
So how many Megapixels or DPI or PPI or whatever is enough for the ordinary genealogist? Of course there is no simply answer because the answer depends on your expectations and your needs (and whether you are the type of person who always buys the most expensive of everything). If you want excellent quality in a scanner, buy up one or two levels. The cheapest scanners now cost much less than $100. Buy a name brand, like Canon or Epson, but move up two levels to the mid-range scanners. You will get speed and quality and dependability. For example, I have mentioned before the Canon 8800 film scanner. This is an excellent product and any comparable name brand at about the same price will do nicely. (I don't get anything from Canon for saying this, by the way).
What about cameras? There are literally hundreds of choices. But here the problem is more difficult. Cheap cameras are cheap because they have lower quality lenses. If you depend on a flat bed scanner for nearly all your archive needs, you don't really have to spend that much on a camera to get adequate quality. But just as with the scanner, you might want to consider buying up a level or two from the consumer point-and-shoot level to what is called the pro-sumer level (i.e. like consumer). These are cameras that are marketed to people who are not professionals but like a little better quality. Unfortunately, there is a huge price jump from the consumer models to the mid-range in cameras. A Canon or Nikon in the mid-range can run between $900 and $1800 dollars with an upgraded lens as compared to a point-and-shoot camera for less than $200. So, you may want to weigh the increase in quality against the big increase in price.
If you have aspirations of becoming a professional photographer, start reading about cameras before buying anything. But, if you are simply going to use the camera for making online images of source documents for your own use, you can get along with a much lower priced camera. Then if you are not satisfied, you can give the camera to your non-genealogist family member and start saving for a more expensive model.
What about printers? Almost every model and manufacture of laser printers has a more than adequate quality of print. The prices are so low, less than $100 in some cases, I find it hard to understand why people buy ink jet printers. There is a special niche in printers, that is, printers designed specifically to print photographs. There are literally hundreds of models of these printers to choose from by manufacturers such as Canon, HP, Kodak, Sony, Epson, Lexmark, Ricoh, Polaroid, and many more. Take your pick, but be aware that the low price is deceiving, the replacement ink cartridges will cost more than the printer. It is extremely difficult to get an accurate idea of exactly how much it will cost to print a picture on any particular printer.
Just a note about Lines per Inch. You will not see anything about LPI in an ad for a digitizing device. Why? Because Lines per Inch is a standard usually confined to the printing trade and high end scanning and digitizing. The definition is somewhat technical. Here is a quote from About.com:
LPI (lines per inch) is an important measurement related to the way printers reproduce photographic images. The LPI is dependent on the output device and the type of paper. Countries using the metric system may use lines per centimeter (L/cm).
To simulate shades of gray using only black ink a printer prints varying sizes and patterns of halftone spots (spots are made up of many dots of ink/toner). Small halftone spots (fewer dots) create the visual illusion of a light gray while larger halftone spots (more dots) appear darker, blacker.
The printer uses a halftone grid divided into cells. The cells contain the halftone spots. How close together the cells in the grid are is measured in lines per inch. This is the LPI or line screen.