Its a mountain peak on the northeast corner of the island of Puerto Rico that dominates the surrounding countryside. It's peak pops in and out of the cloud cover that streams overhead dropping frequent rains that give it the title of the only tropical rain forest in the US Forest Service system.
Its not a National Park, so there's no fee to enter and there's no guard hut at the entrance,there's just a sign though there is also a visitor center near the entrance on the only road in. That would be Highway 191 which is barely marked at the turn off on the major Highway 3 which runs from Fajardo to San Juan through the snowbird condos of Luquillo. The southern entrance to the Forest has apparently been closed for years by landslides so you can only get in from the north, and I am guessing that in winter the place is a raging zoo. In November, even on Veterans weekend it wasn't crowded at all.
This is a place of abundant flora, eg: bamboo, extra large of course, on the road in.
We discovered a place that was fantastic beyond our wildest dreams. It had a magical quality that wasn't the least bit expected. The roadway is a series of undulating curves winding around the hillside, with ample pullouts to allow riders to stop and check out the waterfalls, still wet even as dry season takes over:
And overgrown fantasy creeks, with my youthful colleague leaping from rock to rock for the perfect picture:
and views across the mountain to the ocean 

This is the Caribbean as it must have been 600 years ago before European development of agriculture and plantations, or the arrival of crisp white rental Toyotas, I daresay. It was, as my wife, a native Californian reluctantly put it, "more awe inspiring than the redwoods." How true.

"Say NO to the trolleys of El Yunque" Conch trains are a bone of contention in Key West so I have some sympathy with the locals on this one. I'd have more if the store owners in the forest treated the dogs better... (gotta let it go!). And then there is the Puerto Rican penchant for just stopping wherever the urge takes you. If you see a fern you want to photograph, stop the car immediately, let the family pour out and start clicking. I got into the spirit of this craziness and we learned to do as the locals do and stop the Corolla anywhere the fancy took us. Our white rental car wasn't the fanciest vehicle in the park by a long chalk.
I would put The Caribbean National Forest of my list of places to see before I die, had I not see it already. And I will advise visitors to bring along some dog food and water for the Satos crouching, waiting hopelessly for their owners to return to collect them...Strays. I just can't let them go.