There are actually many different kinds of injections used to control or relieve painful conditions. You may have to ask for more specific information to find out what's being used and why.
It's possible she is receiving acupuncture. Acupuncture is a Chinese practice that has been around for centuries. It's been used for everything from hangovers to allergies to headaches and other body pains. Tiny needles are placed along specific pathways called meridians. The goal is to release energy for healing and to restore normal function. It has few negative side effects.
Prolotherapy is another form of injection used with back pain patients. A solution is injected into the ligaments around the spine and pelvis. The idea is to cause growth of tissue. This could be done through an inflammatory reaction. The result may be to form new ligament and tendon tissue alongside the body's natural ligaments and tendons. In some cases, it may just cause scar tissue to form. Either way, the area is "tightened up". This gives the patient increased stability and may reduce pain.
Steroid injections are often used to interrupt the pain-spasm cycle by numbing the area and reducing inflammation. These injections can be placed inside the spinal canal or inside joints. Only a limited number can be given due to long-term side effects.
One last type of injection are trigger point injections. Trigger points (TrPs) are spots in the muscle and fascia that are overly irritated. TrP injections are usually combined with stretching exercises. Getting rid of the trigger points seems to alleviate the symptoms as well. The number of injections must be limited because local muscle damage from scarring can lead to more problems.