Aspect ratio is the relationship of the width of an image to its height, of its horizontal to vertical dimensions. Some digital cameras let you select an aspect ratio that matches standard print sizes, such as 4x6." For digital cameras without this setting, you'll have to crop images to the desired aspect ratio with editing software.
If a photo doesn't have the the same aspect ratio as the paper it's printed on, one of two things happen. Prints will have an odd sized border on some sides or the printer will arbitrarily crop. So you be in control by cropping a photo before printing it.
All editing programs have tools for cropping at various aspect ratios. Few have as many pre-set sizes as FastStone Image Viewer (illustrated), an image browser and editor that is free for home users.
To maintain high visual quality when cropping images, shoot at the highest resolution and quality settings.
