Before beginning to understand the difference between PPI and DPI, you first need to know what pixels are. Pixels are the small selection of screen which displays a color. Many pixels are combined to create an overall image. Each pixel only contains one color, which is easy to visualize with an 8-Bit drawing. PPI and DPI are very confusing subjects, and there is a lot of false information out there.
PPI, or pixels per inch, is a pretty straightforward term. It refers to the numbr of pixels there are in a square inch of the document. The more pixels a drawing contains, the more color variation there can be.
To learn about why you don't necessarily have to change the PPI of your image to 72 PPI before uploading to the web, check out this website.It is still recommended in many cases to keep your resolution at this size when uploading to the web, because the file size will be smaller.
Keep in mind that your computer is set to a certain PPI. You cannot change the PPI of your computer, because it is determined by the amount the amount of physical pixels your screen can display.
PPI is mostly only important when it comes to printing.
DPI, or dots per square inch, is not the same as PPI. DPI is only relevant when it comes to printing. Printers print images made out of tiny dots of four colors, which are black, cyan, magenta, and yellow. The printer places each of these dots near each other to achieve the colors of the printed image. DPI measures how closely these dots are placed, ie, their density.
DPI is not important to you unless you are planning on printing your image. If you want to print, you should look up what the suggested minimum DPI for quality prints are on your printer, as every printer uses slightly different dots. When you print an image with a higher DPI, your print will take longer and use more ink, because it is making more dots.
Choosing Your Image Size For Printing
Alright, so at this point, you probably have a vague understanding of what is happening with your pixels and dots. It's okay to be confused, this is hard for many professionals in the industry!
You are probably thinking "Well, then does it even matter what size I print my image?" The answer is YES!
When you print an image with too low resolution for that image size, it does not look good. This is common knowledge, but here you will learn how to fix it. Generally, it is accepted that you should print your image at 300 PPI or higher. This article explains the mechanics behind why, and you can read it if you are interested.
Locate your image size box and discover how to change the resolution of your image. You should not see a change on your screen, but if you print your image, you will.
If you have the ability to print an image (it can be black and white if you do not want to use your color ink), print once with a DPI far smaller than the size you will be printing at, and then with a PPI above 300. Post these images and describe the difference.
If you do not have the ability to print, take screenshots of you changing the image resolution in your program. Describe what would happen if you were to print the images.
Describe the process you went through to change the resolution in your particular program so that if someone else is struggling to figure it out, they can use your writing as reference.
Read this article about document size in relation to resolution and printing. Describe, through screenshots and words, how the optimal printing size of your image was changed by the PPI.