There are a few different types of cleanings. The health of your gums determines which one is best for your needs.
For people who have generally healthy gums or gingivitis. In this type of cleaning, the parts of the teeth above the gumline get cleaned and polished. You do not need to be numb for this, and it only takes one visit. If you fall into this category, your teeth should be cleaned every six months.
This type of cleaning is for people whose gums are unhealthy, who have a disease called periodontitis. Periodontitis can range in severity from mild to moderate to severe; it is a condition where the bone that holds the teeth in is pulling away from the teeth. It is caused by not flossing regularly and by not getting regular cleanings every 6 months. Tartar accumulates between and around the teeth underneath the gums and causes irritation, much like having a splinter in your hand. Until the splinter (or in this case, tartar) is removed, your body cannot begin to heal. This is what a deep cleaning does. It usually takes two visits, one visit for the right side of your mouth, another for the left. You do need to be numb for this procedure, as it will keep your more comfortable while the dentist or hygienist is cleaning your teeth.
If you get a deep cleaning, you want to make sure your gums and teeth stay healthy, so you get cleanings more frequently, every 3-4 months instead of every six months. These are called maintenance cleanings. At this visit, the health of your gums will also be monitored to make sure things are improving.