For a romance YA novel, how explicit can it be? Are there certain descriptions or words that shouldn't be used?
There's a good thread on this over at Absolute Write - http://www.absolutewrite.com/forums/showthread.php?t=184551
The general consensus there seems to be that there is definitely a limit on how explicit YA sex should be. They suggest going for the more 'poetic' approach rather than an explicit or erotic one, and I think that makes sense.
ETA:I didn't see any sign of a list of 'banned' words, or anything, and I wouldn't think there would be. Context is probably going to make a lot of difference in something like this. As long as you're writing to serve the story rather than to titillate, I think you'll be fine. If editors don't like something specific, they can let you know. (apparently one writer's editor insisted that in YA sex, there must ALWAYS be mention of a condom.)
There is no hard set rule, and the boundaries are constantly expanding. Tender Morsels deals with incest, molestation, gang rape, abortions, and many other extremely adult issues. But it does so with implications and metaphors more often than gritty details (don't get me wrong, the book is amazing).
So many of the issues could be glossed over by a reader who does not have a seed of awareness towards them. But one who is "ready" to meet or beginning to grasp the issue will be able read between the lines.
I've published quite a few novels for younger readers but my Y/A novel Gobshite, despite being enjoyed and even admired by most of the editors who read it, was declared too near the knuckle for publication -however, it did get me the contact I needed to publish my first series for younger readers, Sebastian Darke.
A good rule of thumb is that YA books can and do deal with edgy or adult topics and subject matter, but NOT in explicit detail. YA books tend to be bought by teens themselves (in contrast to MG books, which are largely bought by parents and teachers, and therefore are typically squeaky clean) which means that they often deal openly with two perennial teenaged obsessions, sex and violence.
Something explicit, however, could potentially be considered pornographic, and I can't imagine any legitimate publisher wanting the controversy or the reputation that would come with putting out a book like that for an underaged audience (especially considering that the protagonists are likely to be in the same age range).