What is Resolution and why do I need to understand it?
Computer images are made of pixels, which are small squares of digital information with a colour value. 
It's how your computer renders a continuous tone image making that image appear to be a nice smooth photograph when, in reality, it's thousands of tiny squares of color all strategical place. A pixel's size will depend on the resolution needed for the application or medium you are working with. With two files of the same dimensions, a file with a resolution of 150 pixels per inch will have larger pixels than a file of 300 pixels per inch.
When discussing pixels, also called "raster" images, it should be referred to as pixels per inch (ppi), not dots per inch (dpi). The image file is made of pixels and the image on the screen is made of pixels. Respectable text books and magazines sometimes talk of dpi here but it is wrong and this term should be reserved particularly for printing. In printing, dots of ink are laid down on paper and the two situations need to be distinguished. So it's ppi for digital images and screen images, and dpi for prints.
The number of pixels per inch needs to be associated with the dimensions of the image in order to give the image resolution. The more data that is recorded by a digital camera or scanner, the higher will be the resolution of the image file. 300ppi in itself is not necessarily a higher resolution than 150ppi; it means nothing until the image dimensions are included.
An image of 150 ppi for an image size of 8" x 10" would be written: 8"x10" @ 150ppi, which is 1200 pixels x 1500 pixels. An image of 300 ppi for an image size of 8" x 10" would be written: 8"x10" @ 300ppi, which is 2400 pixels x 3000 pixels.
The relationship of pixels to area might become clearer if we compare it to tiling a floor. With tiles half a metre square, a floor area of 2 metres by 2 metres would require 16 tiles. An area of 4 metres by 4 metres would require 64 tiles, four times as many. Pixels and files sizes work the same way.
As well as the resolution, how an image looks will also depend on the viewing distance. A small image might require a high resolution because it is viewed closely and hence examined more closely. A street billboard is likely to be of lower resolution because it is seen from a greater distance and printed on quite a coarse screen. When an image is examined from a distance closer than normal, the pixels become noticeable and the image is said to pixelate.
A computer monitor also has a resolution, such as 800 x 600 or 1024 x 768, measured in pixels. If an image has a pixel dimension of 800 x 600 then it will fit on to a 800 x 600 screen whereas it will appear small on a screen set at 1024 x 768. A PC monitor displays at 96ppi and a Mac monitor displays at 72ppi, although some of the newer Mac screens are at 100ppi. A file of 300ppi @ 4" x 5" will have a pixel dimension of 1200 x 1500. When displayed at 100% on a 1024 x 768 monitor, the screen will not be able to show the whole image at once and PC and Mac monitors will view the image slightly differently. This is why when designing a web site, all dimensions of images and text must be in pixels and not inches, otherwise the layout will go haywire when viewed on the other platform. Standard resolution of images for press printing is 300ppi. This allows printing at 150 lines per inch, which is considered standard for most general printing. For more about print resolution, click here.
