Mountain Bike Wheels are available in three sizes 26” (industry standard until a few years ago), 27.5” (also known as 650b which is the newest size to hit the market) or 29” (29er) and can only be used with bikes which are built around that certain diameter wheel size. The next thing to take into consideration is whether you need disc or no disc wheels. Disc wheelsets have fittings on the hub to mount either 6-Bolt or Centre-Lock rotors for disc brakes while non disc wheelsets are traditional hubs and rims where the brake blocks stop against the rim. Rims can be tubeless, ie run without tubes where a special sealant must be used, but most are standard where a tube is used with the tyre.
Knowing what type of riding you will be mostly be doing will determine what width of rim you need. If you are riding downhill (DH), dirt jumping, enduro or more aggressive cross country then a wider rim will be needed. If you are racing cross country (XC) or are only using the bike on light trails then you will usually get away with a thinner rim as the bike won’t be suffering as many big hits. This has an effect on weight with XC wheelsets being lighter than more robust DH wheelsets where longevity wins over weight saving.