Resolution is another key term that is often confused. There are two main acronyms used when dealing with resolution: DPI and PPI.
DPI is only of concern when you're creating work for printed output. It stands for 'dots per inch' and refers to the number of dots per inch on a printed page. Generally, the more dots per inch, the better quality the image. 300DPI is the standard for printing images.
PPI stands for 'pixels per inch' and, as you'd expect, refers to the number of pixels per inch in your image. If you make an image larger in Photoshop, you will increase the number of pixels per inch (with Photoshop making up the data) and you will lose quality.