What are Pixels?

The word "pixels" is short for the term "Picture Elements". Pixels are the building blocks of digital images, which are comprised of tiny, square, individual dots of light. In Photoshop, image sizes are usually expressed in pixels.

The resolution of an image is expressed in the number of pixels per inch (ppi), or dots per inch (dpi). The higher the number, the higher the quality of the image. Your computer monitor is only capable of displaying 72 ppi, so images for the web do not need to have a resolution higher than that. However, documents created in Photoshop that will be printed need to have a much higher resolution. Professional printers often require a minimum resolution of 300 ppi.

Keep in mind also that images are usually device dependent. This means that what you see on your computer monitor probably will not look exactly the same on someone else's computer monitor. Ditto for printer output. An understanding of how Photoshop manages color information is essential to gaining consistency over color across different electronic devices and forms of media.