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What is Dental abscess and explain in detail?
A dental abscess is a localized collection of pus associated with a tooth. The most common type of dental abscess is a periapical abscess, and the second most common is a periodontal abscess. In a periapical abscess, usually the origin is a bacterial infection that has accumulated in the soft, often dead, pulp of the tooth. This can be caused by tooth decay, broken teeth or extensive periodontal disease (or combinations of these factors). A failed root canal treatment may also create a similar abscess.
A dental abscess is a type of odontogenic infection, although commonly the latter term is applied to an infection which has spread outside the local region around the causative tooth.

Classification
The main types of dental abscess are:

Periapical abscess: The result of a chronic, localized infection located at the tip, or apex, of the root of a tooth.
Periodontal abscess: begins in a periodontal pocket (see: periodontal abscess)
Gingival abscess: involving only the gum tissue, without affecting either the tooth or the periodontal ligament (see: periodontal abscess)
Pericoronal abscess: involving the soft tissues surrounding the crown of a tooth (see: Pericoronitis)
Combined periodontic-endodontic abscess: a situation in which a periapical abscess and a periodontal abscess have combined (see: Combined periodontic-endodontic lesions).

Signs and symptoms
The pain is continuous and may be described as extreme, growing, sharp, shooting, or throbbing. Putting pressure or warmth on the tooth may induce extreme pain. The area may be sensitive to touch and possibly swollen as well.  This swelling may be present at either the base of the tooth, the gum, and/or the cheek, and sometimes can be reduced by applying ice packs.
An acute abscess may be painless but still have a swelling present on the gum. It is important to get anything that presents like this checked by a dental professional as it may become chronic later.
In some cases, a tooth abscess may perforate bone and start draining into the surrounding tissues creating local facial swelling. In some cases, the lymph glands in the neck will become swollen and tender in response to the infection. It may even feel like a migraine as the pain can transfer from the infected area. The pain does not normally transfer across the face, only upwards or downwards as the nerves that serve each side of the face are separate.
Severe aching and discomfort on the side of the face where the tooth is infected is also fairly common, with the tooth itself becoming unbearable to touch due to extreme amounts of pain.

Complications
If left untreated, a severe tooth abscess may become large enough to perforate bone and extend into the soft tissue eventually becoming osteomyelitis and cellulitis respectively. From there it follows the path of least resistance and may spread either internally or externally. The path of the infection is influenced by such things as the location of the infected tooth and the thickness of the bone, muscle and fascia attachments.
External drainage may begin as a boil which bursts allowing pus drainage from the abscess, intraorally (usually through the gum) or extraorally