bestphotolessons

If you have an image open in Photoshop and go to Image Size, you'll see two areas for dimensions - pixel dimensions and document size.

In these, there are three sets of numbers to pay attention to
1 - dimensions in pixels with height and width - which is the number of pixels available to use
2 - dimensions in inches (or millimeters, or whatever units you use) with height and width (this is determined by the number of pixels available divided by resolution)
3 - resolution (pixels or "dots" per inch.)

A pixel is the smallest piece of information in an image. Images are made up of thousands to millions of these pixels. DPI or dots per inch is how many pixels are in one inch of the image. This is a highly flexible measurement. If an image is 1800 pixels wide, and the resolution is 300 DPI - then the image is 6 inches across (1800 divided by 300 = 6). If you change the image to 100 DPI but keep the 1800 pixels, then the image is 18 inches across (1800 divided by 100 = 18).

However if "resample image" is turned on (at the bottom of the image size window), and you change the document width from 6 inches to 2 inches with the resolution staying at 300 DPI, the number of pixels across will change from 1800 to 600 (2x300=600).

If the document is 6 inches wide and you change the resolution from 300 to 72 DPI while "resample image" is turned on - the number of pixels will change from 1800 to 432 (6x72=432).

I look at resolution of images differently for print and for web.

For print, I want 300 DPI at however many inches it will be printed at - usually 4x6 inches up to 9x12 inches.

For onscreen use, I make the resolution 72 DPI and want the image around 500 or 600 pixels on the longest edge. I don't really care about inches for onscreen use - most screens are 800x600 pixels and up - so I think in pixels for onscreen use. Your use for the image size may be different depending on what you need - but the same principals are used.