It's a center of King Abdul Aziz University, where environmental research done. You can find all types of researches from cows to bees, from australian desert plants to flowers.
For more pics, see my travelogue.
No entry fees, but arrange with your tourism agency.
It takes 1:30 hour to get there, if you drive from Jeddah.
If you decide to come by a taxi, it's SR20 per person. Better to negotiate with your driver for waiting price. No return taxis there.