When creating designs for print we’ll usually need to use a 300dpi resolution document. We might say “Make it 297x210mm at 300dpi“, so you would make a 300ppi A4 document (notice that misuse of DPI and PPI). If you accidentally created this document at 72ppi, you would have to start from scratch and recreate the design because you can’t magically generate the extra pixels you require out of thin air. If you DID change the resolution from 72 to 300, Photoshop would GUESS what color pixels to insert, but as you can imagine the end product isn’t going to look too good. This is known as Resampling. Sometimes you’ll be asked to create a print document at 150ppi. This is usually the case when working with large format graphics that will only be seen from a long distance, so the close up quality doesn’t matter so much. The lower resolution makes life easier for your computer, and results in a much smaller file size.
If you’re creating a super large document (like a billboard), we might ask you to create the artwork at smaller dimensions, but at a higher resolution (say, 600ppi). This again helps because the smaller document is going to mean smaller filesizes. We can resize the artwork in order to bump it up to the full dimensions, using that original 600ppi resolution to create a larger document at 150dpi.
Using the correct image resolution is vital to creating the best printed item as possible. The higher the resolution, the better it will look. Remember to create and save your documents at 300dpi whenever possible. It makes our life easier, and make your business card look better.