Why are there different wheel sizes, and what do they mean?

Yes, Mountain bike tires come in three common sizes: 26”, 27.5”, and 29” (often referred to as a 29er). 26” wheels are the most common wheel size for mountain bikes. 29" have gained a lot of popularity, as they “roll over” small objects easier. The 27.5" offers the best of both the 26” and 29” wheels.