Read Committed is the default isolation level in Postgres. When a transaction runs on this isolation level, a SELECT query sees only data committed before the query began and never sees either uncommitted data or changes committed during query execution by concurrent transactions. (However, the SELECT does see the effects of previous updates executed within this same transaction, even though they are not yet committed.) Notice that two successive SELECTs can see different data, even though they are within a single transaction, when other transactions commit changes during execution of the first SELECT.
If a target row found by a query while executing an UPDATE statement (or DELETE or SELECT FOR UPDATE) has already been updated by a concurrent uncommitted transaction then the second transaction that tries to update this row will wait for the other transaction to commit or rollback. In the case of rollback, the waiting transaction can proceed to change the row. In the case of commit (and if the row still exists; i.e. was not deleted by the other transaction), the query will be re-executed for this row to check that the new row version still satisfies the query search condition. If the new row version satisfies the query search condition then the row will be updated (or deleted or marked for update). Note that the starting point for the update will be the new row version; moreover, after the update the doubly-updated row is visible to subsequent SELECTs in the current transaction. Thus, the current transaction is able to see the effects of the other transaction for this specific row.
The partial transaction isolation provided by Read Committed level is adequate for many applications, and this level is fast and simple to use. However, for applications that do complex queries and updates, it may be necessary to guarantee a more rigorously consistent view of the database than Read Committed level provides.