Image- image size. if you have resample checked and you change the ppi from say 100 to 300,you will be adding pixels that were not there originally. leaving it unchecked you will see your dimension change to reflect the number you enter in the ppi box and no new pixels will be added.
Are you setting the resolution before doing the artwork? If so, set it in the New Document dialog. This is the best way to go.
If you are changing resolution of existing artwork in Image Size, be aware that when you increase the resolution with Resampling OFF, the dimensions of the image will become smaller. If you decrease resolution the image dimensions will increase (measured in pixels).
If you do it with Resampling ON, as Donald says, you will be changing the overall number of pixels, either making new ones or throwing some away. Resampling either way can degrade the quality of your image.