These days most decent Cross Country (XC) bikes come with an air rear shock. Although many low end models are junk, and can not be upgraded easily by switching out the internals and spring like you can do with a coil model.
Obviously the flip side is air shocks are lighter, and these days they mimic a coil shock very closely. The downside is they are more susceptible to damage from dust, dirt, sand and debris then their cousin the coil shock, and should be properly cleaned and receive the correct maintenance recommended by the your owners manual as stated.
There are many different mountain bike rear shocks available now that include various combinations rebound, compression, sprint rate, air, bottom out and lockout.
Also there are now options for riders that more abusive, or weigh more. Some mountain bike rear shocks with air chambers are built for free riding (FR) or downhill (DH) and the shock can weigh as much as a coil shock. This can be good if you ride XC and are very large and / or aggressive rider and need a bit more strength. Some times adding a drop of extra weight can be the difference between a shock that 6lasts you 3 years instead of 6 months before it blows out.
If you are shopping, remember there are sizing differences, which are based on the eye to eye measurement of the shock. Be careful to order the correct model for your bike.
Mountain Bike Rear Shocks