Is this light stripe at the top of images from my DSLR a light leak?

A couple of "debugging" suggestions.

1. Do you have a second lens? Try mounting that and taking photos. Did the light defect go away?
2. Can you see the defect in live view?
3. If it's possible to take a shot with the body cap on without damage (someone please confirm this), perhaps put the body cap on, tape the viewfinder, go into a darkened room, shine a light in various area in the front while taking a shot. Try shining the light at where the lens and body meets.
4. In the darkened room, shine a light into the viewfinder and take a shot. Did the defect get worse?

If a different lens works fine, then it's probably the lens. The others are meant to help you get more information on where this problem might come from.

tell me whats happening

Considering that the White Thing seems to affect an entire band at the top of the image, I don't think it's a light leak. A leak that happened to shine evenly across the entire width of the sensor would be hard to explain.

Instead, it looks like the shutter is getting a little bit stuck near the end of its travel across the sensor. Remember that the image is inverted on the sensor, so the objects near the top of the scene end up at the bottom of the sensor. The opening in the shutter travels from the top edge of the sensor to its bottom edge, and it looks like it slows down a bit near the bottom, causing a slightly longer exposure in that region.

It might be that there's just some dirt in the mechanism that's impeding the shutter at that point, or it might be that the shutter mechanism is starting to wear out and may need to be replaced. Either way, this isn't a repair that you can probably do yourself -- send it in for service.

Update: If the problem goes away when you use the flash, that's consistent with the sticky shutter hypothesis. Light from the flash is obviously very bright, with a very short duration. You use the flash when there's not enough ambient light to get a good exposure. So, when you use the flash, almost all of the exposure happens in the few milliseconds when the flash fires. The shutter still sticks, but the extra exposure that happens at that point makes very little difference because the exposure is determined mainly by the much brighter flash.

Just looking at the shutter won't necessarily tell you anything. The shutter curtains themselves may be in perfect shape; the problem seems to be that something is slowing the second curtain down as it comes to the end of its travel.

You don't have a light leak in your camera body.

Your examples demonstrate the classic symptom of a slow second shutter curtain.

Your second shutter curtain is sticking at the end of it's transit across the sensor.

The fix may require only a bit of cleaning and lubrication, or it may require a new shutter assembly. Unfortunately, a shutter replacement will almost certainly cost more than a D90 is currently worth.

This looks like a faulty shutter to me.

Remove the lens and look behind the reflex mirror. The shutter should consist of a set of curtains which cover the sensor. They should be straight from left to right. If any of these curtains are damaged or out of place, you'll need to get your camera serviced. (Nikon service would probably just replace the shutter assembly, which probably isn't worth the cost.)

I'm pretty sure your camera body does not have a light leak. The Sony A7R had a design flaw causing light leaks and it did not look like a horizontal streak.