Delayed ejaculation is when a guy can usually get a rock hard erection and have intercourse for a really long time, but can't ejaculate. What's particularly fascinating is that the majority of men with this problem are able to ejaculate just fine when they masturbate. It's when you put a flesh-and-blood partner between the guy's hand and his penis that he comes slower than a slug on sandpaper.
Delayed ejaculation used to be known as retarded ejaculation, until we decided that calling a man a "retarded ejaculator" was a bit harsh. It's also referred to as inhibited ejaculation. Even though somewhere between 2% to 6% of men have the problem, very little research has been done on it and very little credible information is available for consumers.
Of course, if you're a man who comes faster than Hans Solo in a Millennium Falcon, you might be thinking, what's the big deal here?--I'll trade my prematurely ejaculating penis for another guy's slow stick in a heartbeat. But unless you've been there and done that, it's hard to understand just how cumbersome and what a burden on a relationship delayed ejaculation can be. It can make sex hard work for both partners.
There can be numerous factors that contribute to how fast or slow a guy will launch his load. So what causes delayed ejaculation in one person might not be the same things that cause it in the next person--and I do think it is "things" rather than thing.
I also want to comment that I think we hold men to a different standard regarding their orgasms than we do women. Not many years ago, we used to call a woman who couldn't have an orgasm from intercourse frigid. I suppose frigid is nicer than retarded, but today we say it's completely normal if a woman doesn't have an orgasm during intercourse, as long as she can pop one out when she's masturbating. Unfortunately, we are neither as kind nor generous with men. If a man can ONLY come from masturbation but not intercourse, we assume there's something wrong. He often feels horrible about himself, and his partner is sure it's because he doesn't find her sexually appealing or she can't do anything good for him in a sexual way.
So if you're dealing with this problem, try to remind yourselves that there are plenty of ways you can enjoy intercourse and sexual intimacy without needing a male ejaculation as reassurance that you've crossed the lovemaking finish line. What if you agree on a signal the woman can give during intercourse for when she's satisfied and wants to stop? This takes the pressure off him, and it takes the pressure off her.
Also, if you are a therapist or a consumer who wants even more depth on the subject, these are the two best articles I think you'll find on delayed ejaculation (I would read both--as I think they are complimentary and more helpful in tandem than on their lonesome):
"Treatment of Delayed Ejaculation" by Uwe Hartmann and Marcel D. Waldinger, which is Chapter 10 in the 4th edition (Nov. 2006) of Sandra Leiblum's highly respected Principles and Practice of Sex Therapy.
"Retarded and Inhibited Ejaculation" by Michael Perelman and David Rowland, which is Chapter 4 in the "Handbook of Sexual and Gender Identity Disorders" by David Rowland and Luca Incrocci (2008).