Platinum requires 75 annual paid nights or more per year. At times, if you're close and there's a history of being close, you can be brought in (I hear), but there seem to be no guarantees.
Platinum Premier is a category within Platinum that's given annually to the top three percent of Platinum members--those normally having over 125 nights paid per year. The major difference is that you have a special phone number and Platinum Premier bennies given to you (such as an annual upgrade at resorts for one night when staying on points).
The Marriott.com web site explains it all.