Here are some various resolution explanations for common scanning/printing occurrences.
200 DPI Documents
200 DPI (Dots Per Inch) is a perfectly acceptable scan for any document. 200 DPI is considered the "lowest acceptable resolution" for a black and white document and is equal to what is called "high fax quality." There's no real reason to worry about compression when it comes to black and white documents, as there would be for color scans.
300 DPI is next, is slightly better and is considered conventional. With the highest being 600, which should be reserved for documents with exceptionally tiny print.
600 DPI Photographs
For color photographs, 600 DPI is a great option, especially if they are being printed. A scan at 600 DPI will ensure that every detail is visible and crisp. 600 DPI is common resolution for laser printers and, for that reason, an 8X10 scan can produce a great/good 16X20 reprint. Giving you an idea of the quality in which 600 DPI produces.
Briefly, for comparison consider the 1,200 DPI scan. While it may seem like you are getting a better quality (and you are in some ways), the visible difference for basic prints won't be stark enough for your eyes to perceive. 600 DPI works just fine for all basic uses and practices.
4000 DPI Slides
All 35mm images have a width of 3 : 2 (1.42 inches wide x 0.95 inches high). So when a slide is scanned in at 4000 DPI (equivalent to 18 megapixels, and the highest possible scan for transparent film), the equation looks like this: 4000 dpi x 1.42 inches = 5680 dots wide (by comparison 2000 looks like this: 2000 dpi x 1.42 inches = 2840 dots wide). When printed on a 300 DPI printer, both look like this:
2840 dots / 300 dpi = 9.5 inches wide
5680 dots / 300 dpi = 18.9 inches wide
Which should you chose? Consider your purpose. Any image that needs blown up to poster size should be scanned at 4000 DPI. If not, then stick to 2000 DPI. Photos that will undergo extensive photo manipulation should be scanned at 4000 DPI. The opposite is true for 2000 DPI.
Please note: Because of the quality of 4000 DPI, you will be able to enlarge the photo many times it original size. A benefit for those looking to crop particular portions out of the picture.