Medicare Prescription Drug Plans: These plans (sometimes called “PDPs”) add drug coverage to Original Medicare, some Medicare Cost Plans, some Medicare Private Fee-for-Service (PFFS) Plans, and Medicare Medical Savings Account (MSA) Plans.
To join this plan an individual must already be using Part A and/or Part B

Medicare Advantage Plans (like an HMO or PPO) are other Medicare health plans that offer Medicare prescription drug coverage. You get all of your Part A and Part B coverage, and prescription drug coverage (Part D), through these plans. Medicare Advantage Plans with prescription drug coverage are sometimes called “MA-PDs.”To join this plan you should be entitled to Part A and Part B and be a resident of the area where you intend to receive the service.

Source: www.medicare.gov
To use a Medicare prescription drug plan Medicare.gov has a tool to search and compare coverage options for a particular area.

About.com Seniors has very good information about prescription drug plan coverage. To find some interesting facts about prescription drugs and seniors visit the site HeinzFamil.org

Prescription drug abuse in seniors is an important factor to be considered. Since they are likely to be on prescribed medicines for a long time, there is always a chance of getting used to them, thereby abusing them. Misusing the drugs in seniors has a very fatal impact on their life. Tranquilizers and sleeping pills are the most common drugs abused by senior citizens. Gradual reduction in the usage of drugs, exercising and counseling are some of the means to handle drug abuse.

Other major factor is unexpected reactions from multiple prescription drugs. It is the fourth leading cause of death in seniors. Below is a video describing drug safety in seniors.