I don't get it. Personally, hate the term pre-K. It always seems to come from someone trying to be snooty and making attempting a distinction between the pre-school year before K and other pre-school years.
I personally think of it has 3-year-old and 4-year-old pre-school. But I'm the odd-ball. There are some pre-schools around here that distinguish:
Parents/Kid Day Out (1-2 years) - 1 day/week
3 year old pre-school - 2 days
4 year old pre-school - 3 or 4 days
pre-k - 4 or 5 days
Most commonly pre-k classes are made up of the summer b-day kids not starting K and fall b-day kids that have already had two years of pre-school.