In the image above, notice the difference between the amount of teeth between the #7F and the #7 blades. The #7F on the left has twice as many teeth as the #7 on the right.
When shopping for clipper blades, you will often see two cutting varieties - Fine and Coarse.
Fine blades (often designated by the "F" in the number) are perfect for shaving down most pet and animal coats. Fine blades are typically the standard blades used across pet grooming.
Coarse blades, however, are skip tooth blades, which means they have fewer teeth across the blade set (see image above for comparison). Because they have fewer teeth, coarse blades are designed to cut better through thick or coarse coats that would usually tend to bog down a regular fine blade.
Please exercise caution when using coarse blades. Because coarse blades are skip tooth blades and have fewer blade teeth, you must be sure to hold skin taught when clipping to avoid any accidental nicks.