Once you turn 65, you're eligible for Medicare, which many veterans of the red tape say is a bewildering maze of forms, insurance terms and medical lingo.

Financial consultant Connacht Cash learned just how confusing it could be when her mother's illness plunged her into the world of Medicare. She learned even more when her father became ill with prostate cancer.

"At least the second time around I understood the medi-speak, the insurance terminology and the legalese," she recalls. "But, again I found an amazing number of things I had to argue with Medicare about and an equally amazing number of things that Medicare didn't want to pay for."

Cash may have had it tough, but she's put her hard-won knowledge to good use in "The Medicare Answer Book" (Race Point Press). In it, she defines just what Medicare is, how to qualify and how to fill out the forms. She also tackles such topics as Medigap policies, managed care and long-term coverage.

"What you don't know about Medicare can affect what kind of treatment is received, where it is given and for how long it continues," she warns readers.

Knowing the difference between Medicare, Medigap and Medicaid will help you determine how much and what type of coverage you need.

Medicare is a federally funded health insurance program for people 65 and older. Here's where it gets tricky: It comes in two parts. Part A covers expenses for treatment at a hospital or skilled nursing facility. Hospice care for a terminal illness and home health care are covered; nursing home care is not.

Part B covers doctor's fees, lab tests and other services not covered by Part A, and you don't have to be a hospital patient. Part A doesn't cost anything once you qualify (It's paid for by Social Security payroll taxes), but there is a monthly premium (currently $43.80) for Part B.

Medigap is a type of insurance policy sold by private insurers and designed to fill the many gaps in Medicare Part A and B coverage. You pay for this out of pocket, and policies vary widely.

Medicaid is a government program aimed at people with limited financial resources.

"Unlike Medicare, eligibility for Medicaid is based on financial need," Cash explains. But since illness often drains financial resources, many elderly Medicare recipients also qualify for Medicaid.

A big part of getting the most out of Medicare, Cash contends, is learning the lingo of insurance. This will help you understand those statements that arrive in the mail, as well as choose the best insurance policy. Here are some, but certainly not all, of the key terms:

*Benefit period: refers to a set time frame used to calculate deductibles, co-payment amounts and total coverage costs. For example, in Medicare Part A, a benefit period begins when you enter the hospital and ends when you've been out for 60 consecutive days.

*Actual charge: what a health-care provider charges you, which is not necessarily the amount Medicare pays. It can be more — sometimes much more. "Approved amount" is the amount Medicare decides it will pay for services; if the physician charges more, you pay the difference (known as an "excess charge").

*Assignment: an arrangement between Medicare and your physician under which the doctor agrees to accept Medicare's approved amount as full payment.

*Medically necessary: services deemed appropriate by Medicare. All expenses covered by Medicare must be accepted as medically necessary.