How do external flashes, like the SB-400 for Nikon, affect pictures?

I have a D3000, and was wondering how something like the SB-400 would affect picture quality? I usually take portraits (both indoor and outdoor), and a lot of low-light pictures. Would the SB-400 help make pictures look nicer? Or do people buy these to get certain affects that I'm not aware of?

The main effect of portraiture with flash is to use bounce flash: turning the flash away from the subject. You turn the flash away from a subject and let it bounce off a roof/wall. The light is much softer. Direct flash makes the image harsh.

I suggest you got for the SB 600 where the head swivels sideways and up. You could still use the SB400 using it as a remote flash with a radio controlled trigger.

The SB400 can improve your image if you are shooting in dimly lit areas indoors. You can tilt the flash head so you can bounce the light off the ceiling (search for "bounce flash") to give a nicer looking image than direct flash. The SB400 is fine for small rooms with low ceilings. I prefer this over my larger SB600 when shooting at small parties since it's a small flash.

Outdoors, you can use the SB400 as a fill flash during sunny days. At close ranges (less than 10 feet) you can use the SB400 to light up (fill in) shadows on faces. Set the flash power (setting in your camera) to something around -1 to -1.7 stops.

Warning about the SB400, the power output of this flash is a bit better than your on-camera flash. This flash doesn't have enough power to use in big rooms. The flash head only rotates in one axis which limits where you can bounce the flash. Better flashes have two-axis head rotation.

The next step up from the SB400 is the SB600 (discontinued, but might find some around still) and the SB700 (newly released). These have a lot more power than the SB400 and have two-axis head rotation. The SB600 & 700 are considerably larger than the SB400.

A single on camera flash may be the worst artificial portrait lighting on the face of the planet.

For portraiture, you need a minimum of two lights. A key and fill light.

Here is a link that will help you figure out how to use lighting, flash or other

http://strobist.blogspot.com/

It is always better to have more light and just tone it down to your taste rather than have less and end up wishing for more.