Instead of cribbing the whole text of the Ms-PL and dropping it here, I'll just refer you to the source:

Open Source at Microsoft: Licenses: Microsoft Public License (Ms-PL)

That hopefully answers the "what" question.

The "why" question is a little more interesting... You need one because:

    1. Your contest entry must be Ms-PL-ed to be considered for a finalist and/or a winner.
    2. You need it to protect your own copyright and intellectual property.
    3. Your users need it so that they can legally use it, grow it and extend it.

Oh, and since it's pretty short, you should probably read it... especially 3(D) which says (paraphrased): "Don't forget to include a copy of the license in your distribution somewhere."

Good luck!!