If an image is saved in TIFF, the quality is the highest and as there is no compression that affects the image quality, the file size is also very big. If an image is saved in JPEG, the quality is not as high as TIFF as it undergoes compression. The image quality is then dependent on the amount of compression. The higher the compression, the smaller the file size, and the lower the image quality. Conversely, a low compression would mean a larger file size, but better image quality.
Image size is basically how big an image is, and is usually expressed as x by y pixels - eg.
1600 x 1200 pixels. This means that there are 1600 horizontal pixels and 1200 vertical pixels. This equates to 1,920,000 effective pixels, which is the common specification for 2 megapixel cameras. Image resolution, on the other hand is used when the image is to be printed or viewed on a monitor. For the example of 300 ppi, this means that 300 pixels are squeezed into a length of 1 inch.