These two acronyms are often used interchangeably although they do have different meanings.
This guide will use only PPI (Pixels Per Inch) to describe resolution.
The physical size of an image refers to the dimensions in which it would print (ex: 8.5" x 11") or the pixels dimensions of an image displayed on the web (ex: 600 pixels x 800 pixels).
The memory size is how much memory an image file takes up on a hard drive. For example, a JPG image saved on a computer may be 2 MB (megabytes), meaning that it will require 2MB of space on a drive to store that image.
Not all of our images are the exact size we need them to be, so it's important to understand how to properly resize an image and how resizing works. When an image is resized, its pixel information is changed. For example, an image is reduced in size, any unneeded pixel information will be discarded by the photo editor (Photoshop). When an image is enlarged, the photo editor must create and add new pixel information -- based on its best guesses -- to achieve a larger size which typically results in either a very pixelated or very soft and blurry looking image.
When working with raster images (pixel-based) it is important to understand that scaling an image in programs, such as Word, Powerpoint, InDesign, or Dreamweaver, does not actually resize the image, but rather stretches images larger or scales them smaller. When scaling, the resolution is not adjusted to best suit the new size, rather the pixels are stretched and can appear pixelated.
The most common side effect of scaling an image larger than its original dimensions is that the image may appear to be very fuzzy or pixelated. Scaling images smaller than the original dimensions does not affect quality as much, but can have other side effects. For example, if you upload a very large image to a website and scale it down to a smaller size, the website still must load the full size version of that image and could cause the web page to load more slowly.