Excerpt from an article about the XD. The whole thing can be found here:
The striker is analogous to a firing pin, for both of them strike the primer and make the gun go bang, but how they do it is different. If we use a system such as the 1911 for comparison, the firing pin is struck by the hammer, which is powered by the mainspring. There is no direct connection between the firing pin and hammer or any other part of the firing mechanism. When the hammer hits the firing pin it is driven forward against the tension of the firing pin spring.
Strikers work differently. The striker is drawn back against the tension of a spring and held there until it's released to fly forward and strike the primer. They've been around in pistols since the dawn of the 20th century and long before that in bolt action rifles. But when we try to attach ordinary definitions they don't always fit very well. Three terms: single-action, double-action and double-action only are used to define how pistols are fired. They are also a mighty source of confusion. Striker firing mechanisms complicate things a bit, and although we try to make them conform to the same definitions they don't exactly fit. I wish we didn't spend so much time trying to stuff things into pigeonholes.
Some self appointed experts have proclaimed striker firing mechanisms are unsafe because you're running around with a cocked single-action gun. Obviously, their knowledge of firearms, vast though it may be, doesn't include knowledge of virtually all center fire rifles and shotguns, which have actions that conform, quite precisely, to the single-action definition. Many of them are striker fired too. Look at bolt action rifles and tell me that the firing pin is not a striker. We routinely go afield with shotguns and rifles that are loaded with the safety engaged and think nothing of it. How then, can a striker fired pistol be more or less safe than a bolt-action rifle or pump shotgun with an operating safety?