What does back surgery involve?

Varies. Most of the time it is done under general anesthesia (put to sleep with drugs), cleansing the skin, making an incision, moving tissues gently out of the way to get to the problem area. Depending on the problem, abnormal tissue may be removed or a fusion (making part of the spine stiff but stable). Sometimes implants of various kinds are used. The size of the surgery varies tremendously.
What kind? Spine surgery can range from outpatient, minimally invasive surgery done with the patient under a local anesthesia to complex, multi staged reconstructive spine surgery requiring close monitoring intraoperatively and post operatively with placement into a rehabilitation facility after leaving the hospital for several weeks or longer.