An easily read discussion of El Nino can be found in Wikipedia - http://en.wikipedia.org/wiki/El_Ni%C3%B1o
There are lots of discussions of El Nino, most of which I found way over my head, but none of them seem to do much for telling people how
1991-1992, 1993, and 1994 were all El Nino years - unusual. We were in the S. Pacific from 1992 through 1995, crossing from Ecuador to Australia in 1992 - 1993. In 1994/1995 we sailed up to Papua New Guinea, east to the Solomons, and then E to Vanuatu and Fiji before turning around and sailing back to Oz in November 1995 (clearly, no gentlemen are we!). With a few exceptions, we did not experience weather as unpleasant in the S. Pacific as what we experienced in the Caribbean.**
We did not suffer from the Queen's Birthday Storm (1994, a horror of a storm that you can read about in "Rescue in the Pacific" by Tony Farrington) - that affected boats leaving New Zealand and sailing up to Tonga or Fiji. Was that part of El Nino? I would say that such a storm is just a freak, as was the "Perfect Storm" in the Atlantic, or the Fastnet disaster in 1976 (?).
The Earth's climate is so complex that it seems meteorologists are hard-pressed to make anything other than very general predictions. However, freaky weather is not constant, and when the weather turns unpleasant, sometimes the prudent choice is to turn back and seek shelter. Our worst trips were when we weren't wise enough to do this - one of our worst crossings was from Western Samoa back to Pago Pago, American Samoa in 1993. Awful, and we were too stubborn to turn around and go back - we're just fortunate that discomfort was all that we suffered.
A friend almost lost his life in a horrible storm on his sail from New Zealand to Australia back in the 80s. It was not an El Nino year.
Storms or less than perfect weather occurs in El Nino years. Storms and less than perfect weather also occur in non El Nino years.
The Pacific is a huge ocean, with long passages, which makes it a bit scary. For many cruisers the passage to the Marquesas is their first long passage (more than two or three days), which I think is amazing. I would suggest that you be prepared for bad weather - practice in shorter hops in all kinds of weather provides a great deal of experience that can't be replaced with weather charts and lots more words on paper. If you have a well-found boat and are confident in your abilities, mentally as well as physically, the bad weather you encounter will most likely be an annoyance, not a survival experience. Then the perfect sails will amaze and delight you.
**We once sailed into Fort-de-France, Martinique and as we passed one of the boats anchored in the harbor, a fellow came into the cockpit and yelled over to us, "where did you come from?" We replied "St. Martin". He then asked how rough a passage had we had, to which we replied, "not at all, it was beautiful!" We had to shrug our shoulders at him when he told us that they had been stuck in port for more than a week because they were told the weather was very bad out there. Well, there was a lot of wind, but not a WHOLE lot of wind compared to what regularly blew in the Caribbean. With a few exceptions, mostly when we were outside the tropics, we found the Pacific winds to be generally less than what we regularly encountered in the Caribbean. So go to the Caribbean and sail. a lot. You'll be prepared for almost anything then.