A 20 year term life insurance policy is one that is for providing a death benefit if the insured passes with in that time frame. Who may be served well with this type of coverage? How about those that want to cover (in the event of death) money borrowed or loan for that time period ie mortgage or to ensure their kids can pay for college (if the bread winer parent passes). Generally as far as premiums go they can remain level (or in other words the same over the life of term of policy) or can be subject to change. For instance annual renewable which be subject to adjustment each year.
For more useful information on life insurance see the video below.