Pulpitis simply means inflammation (-itis) of the dental pulp and is a very common cause of toothache. The dental pulp is the innermost part of a tooth which contains its blood and nerve supply.
Broadly pulpitis can broadly be broken down to Reversible and Irreversible Pulpitis
As the names suggests reversible pulpitis is less serious in severity of symptoms and the nerve can generally be returned to full health. Reversible pulpitis is the more serious version and leads to death of the pulp and if left untreated can lead to a dental abscess.
The symptoms of each type of pulpitis differ but there can be a certain amount of crossover between the 2.