Bokuto or Bokken?

What is a bokuto? What is a Bokken? What's the differnece?

They're the same thing. A bokuto/bokken is a wooden sword simulator used for safe kenjutsu practice with sword on sword contact. 

Bokuto (木刀)  has two chatacters:  "boku" meaning "wood" and "tou" meaning "sword" (or "blade", or "knife").
Bokken (木剣)  has two chatacters:  "boku" meaning "wood" and "ken" meaning "sword". "Bokuken" gets contacted to "bokken".

The terms are synonymous. The use of one over the other is peronal preference and somewhat subject to region and context. In the koryu context "bokuto" tends to be more common, and in the gendai arts like kendo "bokken" tends to be more common but it is not a rule.  In Canada, and I assume the English speaking world, people are more familair with the term "bokken". I can only assume that this is due to the popularity of Kendo.

For a bit more on what makes a a good bokuto and where to get one, go here.