DPI stands for Dots Per Inch and is the resolution printing machines output at.
When designing artwork your concern should be PPI - Pixels Per Inch, the majority of images taken from the web will be 72ppi, however this quality resolution will not be high enough to give you a good printed result. We recommend a minimum of 350ppi up to 600ppi for optimum quality.
Please note that opening a low resolution 72ppi image in Photoshop and simply increasing it to 350ppi will not increase the quality of the image, you need to open the original (native) file then increase the resolution to your desired PPI.
The example below shows the difference in printed quality of a 72ppi image (low screen resolution) vs 350ppi high-resolution printable image.