Alarmed that medical students (even to be) don't know what a deductible is. Don't you people have to know a bit about health care funding for your interviews?
Anyway: a co-pay is a fixed amount that you pay for a service. Typically plans have co-pays for office visits and medications. Just like the $20 co-pay mentioned above.
The deductible is how much out of pocket expenses you pay each year before your "co-insurance" kicks in. So, you pay the first $500 (or $2000) out of pocket for any and all expenses and then the insurance companies covers 80% of everything up to 1 million (or 75% to 2 million or 50%...you get it?).
Before the days of HMOs, most people had only a deductible based insurance policy (no co-pays). Now, more people have programs with no deductible, but a co-pay for every service.
In my opinion, if you are a person in general good health, in your 20s, it is a good idea to have a cheap, high deductible policy. What that means is that you are taking the risk that you may get suddenly saddled with $500 of medical bills (or 1K) but saving a lot each month. Nobody wants a few thou of medical bills, but the risk that will happen is low. Save money and assume the risk yourself instead of paying $200 a month for services you will not be likely to use. (This wouldn't apply if you had/have a chronic illness, frequent sports injuries, etc.)
In my state you can get a $1000 deductible policy on a 25 year old male for around 60/month.