A very basic isolation level which despite its theoretical shortcoming is the default in many database systems. Read committed usually allows for greater performance while sacrificing for a plethora of anomalies.
The actual concrete guarantees in database systems which advertise as read committed are often stronger than the absolute minimum theoretically required.
An isolation level as well as a method with many desirable properties. This model and its anomalies are quite easy to reason about since every transaction operates on a stable snapshot and conflicts will only materialize when write sets overlap.
A property of methods implementing snapshot isolation is that readers do not block writers and writers do not block readers. Only overlapping write sets will cause conflicts and lead to aborts during the validation phase.
The gold standard of transactional isolation. All transactional anomalies are avoided.