Because of a race condition, when several transactions concurrently read from and write to a file, variable or database, the following data problems called phenomena can arise:
The isolation levels are defined in terms of this phenomena (problem) that must be prevented between concurrently executing transactions.
A transaction reads data that has been written by another transaction that has not been committed yet.
See Dirty read
A user queries a row and then later queries the same row, only to discover that the data has changed.
A transaction reruns a query returning a set of rows that satisfies a search condition and finds that another committed transaction has inserted additional rows that satisfy the condition.
See Dirty Write