This topic is a pretty big can of worms for most photographers. There is a lot of nomenclature confusion in this area. I'll break this up into eight areas that should make things a little clearer. I'm using these terms as they relate to what we do in Photoshop.
I know this is a little confusing and you are probably wondering,"Do I really need to know all of this?".The answer is "yes" if you want to know your options when capturing, viewing, and printing digital files. To be honest, most people don't really understand this stuff. They take a picture and hope that it prints out OK. Personally, I don't like leaving things that are important to me left to chance. I like knowing something is or isn't going to work before I try it.
Pixel Dimensions - This is the number of pixels that make up the image file . Usually determined by the number of pixels that make up your cameras sensor. A five megapixel camera has a sensor that is composed of approximately five million pixels (little squares). The more pixels, the more detail the sensor can capture. If you multiply the pixel width by the pixel height above, you come up with 9980928 - pretty close to 10 million pixels. This is from my wife's 10 megapixel camera.The more megapixels the larger print you can make without seeing the little squares (pixels) that make up the image.
Pixel Size - A pixel is not a unit of measurement. It is the smallest picture element (square) in the digital file. The pixel can be any size when printed.The printed size of the pixel is determined by the image resolution in the document size section above. You adjust this in Photoshop.
Image Resolution - The image resolution is how many pixels you choose to have in a particular area - usually in an inch. The example above is 3648 wide. If you put 180 of them in an inch it would create a printed document that is approximately 20 inches wide. If you put 72 of them in an inch the document would be approximately 50 inches wide. If you put 360 of them in an inch, the printed document would be approximately 10 inches wide.You aren't changing the number of pixels by doing this.You are just putting a different amount of the same 3648 in a given area.
Document Size - This is the printed document width, height and resolution.
Image Size - The umbrella that covers pixel dimensions and document size.
File Size - How much space a digital image takes up on storage media (hard drive, memory card, etc.).In the example above it is listed at the top as 28.6m. The M stands for megs.
Image Resizing - Image resizing is just redistributing the original pixels. This is what I'm talking about above.You can resize all you want and the pixel will still have its original quality. Always try to resize if you can. Always be sure the Resample Image box is not selected
Image Resampling - With this box check, you are not just redistributing the pixels you have. You can create now one or throw some of the originals pixels away. Throwing away pixels will usually be necessary if you need to prepare the image to be displayed on a Web page, but once the original pixels is gone, you can't get them back. If you create more pixels by resampling Photoshop does something call interpolation.It takes the real pixels, spreads them apart (how much they get spread apart is determined by how many pixels you are adding) and makes up the new ones based on what it sees in the real pixels.THIS DOES NOT WORK WELL. Fake pixels are not as good as real pixels. This will make your image look soft and generate digital artifacts that will not look as good as the original.That said, sometimes you need a bigger file than you have, so you need to do this. Do it as little as necessary.
Read the information again and ask me questions if something isn't clear. You need to understand this stuff, not just read it.