In determining the amount of a realized gain or loss to be recognized in the current year,

Question:

In determining the amount of a realized gain or loss to be recognized in the current year, certain types of gains and losses are deferred, whereas others are disallowed.
What is the difference between deferring a gain or loss realized in the current period and disallowing the recognition of a current period loss? Give at least one example of each that has been studied to date in this course.