Taylor - your D90 has a maximum flash sync of 1/200th sec. It's usually a setting within the camera or a setting particular to a mode that limits the sync speed to a lower setting. This can be changed in-camera in some cases.
Basically, you're sensor has to be exposed by the movement of two curtains (the shutter) that pass over the sensor. One curtain moves down when you press the shutter release, exposing the sensor initially. Then the second curtain moves down, covering the sensor. This second curtain is what controls the length of the exposure.
The problems with flash is that even though the burst of a flash is very, very fast (into the thousandths of a second) the whole of the sensor must be uncovered between the two curtains for the flash to register across every part of the sensor. When using faster shutter speeds, before the first curtain has fully exposed the sensor, the second curtain is moving down so when the flash goes off, only a portion of the sensor is exposed. This results in a black band across the image where the flash burst hasn't 'touched' the sensor.
Manufacturers have worked around this by introducing a high-speed sync (Canon calls it HSS, Nikon calls it FP Sync), which is a series of bursts that are invisible to the eye. These bursts are basically a percentage of the overall flash output so as that thin gap between the two curtains passes over the sensor, the bursts follow each other so the sensor receives an equal amounts of light, even at high shutter speeds. The downside of this is because the flash output is chopped up to create these bursts, the flash can't illuminate as far.
HSS comes in handy when you want to retain a very wide aperture (say f/2) in bright conditions when you can only get high shutter speeds. But of course, the power of the flash can't illuminate as much so many people reserve HSS for situations where fill flash is required, and not when they want to overpower the sun.
Remember, even at a slow shutter speed, a burst of flash will freeze action and it is the aperture that controls the flash exposure, not the shutter speed. The shutter speed controls the ambient exposure.
This is a brilliant explanation of the relationship between shutter speed and aperture when using flash -
http://strobist.blogspot.com/2007/09/lighting-102-33-balancing-flashambient.html
It's worth looking at the '
Lighting: 101' section on Strobist to learn about flash in general.
