For all intents and purposes, you are buying tickets by the "day." There is no such thing as an "EPCOT ticket" or a "MK ticket."
So first, you need to decide how many days you want to be in a Disney park during your vacation... don't think right away about which park that is or how many parks to visit each day... just how many days you want to be in a Disney park during your trip.
Now that you know that number, think about whether you want to be in MORE THAN ONE Disney park on any of those days. If you prefer to be able to travel from park to park during the day, then you'll need the park hopper option.
If not, you can just buy "base tickets." These allow you to pick a park each day... once you pick that park, you're essentially "tied to it" for that day. You can sneak out for a nap at the hotel room, but if you want to go back to the park, you must go to the same park you're "tied" to.
Your tickets will expire within two weeks of the first use, but that probably won't affect you on your short trip. However, some people prefer to take advantage of discounts and buy for multiple trips at once, opting to pay more for non-expiring tickets.
These guidelines apply to Magic Kingdom, EPCOT, Disney-MGM Studios, and Animal Kingdom. If you want to go to water parks or other Disney destinations, there are separate tickets to be purchased.
Hope this helps, and welcome to the boards!
PS - If you get completely confused, might I suggest you contact some of the great boardmembers from Kingdom Konsultant? They can guide you through the process and assist with other travel needs (for no charge to you). I personally use "wannabebelle" (marie@kingdomkonsultant.com) and highly recommend her or any member of the KK staff.