Dental anaesthetic is used when a dental procedure may cause pain. Because of the sensitivity of the gum many simple procedures such as a simple cleaning can cause pain, but a dentist won't generally use an anaesthetic in these occasions as it's not really warranted. However local anaesthetic is used on a daily basis in dental surgeries, for simple procedures such as a root canal or abscess treatment, but for more painful procedures such as wisdom tooth removal a general anaesthetic will be used.

Levels or strengths of anaesthesia.
With information from you your dentist will decide what level of aneasthetic to administer you. A patient with a condition such asphobia or mental retardation may require higher levels of anaesthetic to help them remain calm through the procedure.

What do anaesthetics do?
A local anaesthetic will numb a specific area where a procedure is to be undertaken, ona specific tooth for example. Generally a topical anaesthetic is applied to the gum where the local anaesthetic will be injected, as the injection itself can be quite uncomfortable and painful. Although if a topical anaesthetic is not used the pain of the injection soon disappears as the local is injected.
Alogn with local anaesthetic a sedatize may be administered to calm the patient, the patient will be calm yet fully responsive throughout. A deeper sedation may be administered where the patient will still be conscious but not responsive. A general anaesthetic is entirely more serious as it puts the patient completely to sleep and is only used as a last resort for large procedures such as wisdom tooth estraction or operations.

If dental anaesthetic safe?
All drugs carry some risk however when administered by a trained anaesthetist they are very safe and the occurence of problems due to the anaesthetic is almost unheard of. However your dentist should ask you about your current and past health before administering anaesthetic, you NEED to tell your dentist about any past problems or allergies. Also of upmost importance is telling your dentist if you are on any medications or are pregnant.