There are all kinds of cheeses in the world; some are softer and other are harder, and some are whiter while others are more yellow.
On the other hand, one quality that some cheeses are known for, especially Gruyere and/or Swiss cheese, are the little (or big) holes that reside throughout their structure, and are most obvious when sliced from the deli.
These holes are just remnants of carbon dioxide gas that were formed as a by-product of the bacteria that makes up cheese. As the curd hardens, these holes solidify, creating the tunnel-like holes seen in cheese as you slice through it.
The holes are not harmful, and many times, some cheeses have them put there on purpose for aesthetic.