ISO on camera DSLR (Nikon)?

I'm going on a trip in a couple of weeks and i will need to take photographs outside, (hopefully) in the sun! What ISO should i use, and can you quickly explain the ISO thing? Like more light=higher or lower ISO?

It's fairly straightforward - the more light, the lower the ISO you can get away with.

If you look at issues of, 'National Geographic,' well into the 1980s, those stunning colour images were often taken on slide film with an ISO of 64!

ISO means sensitivity. In good light you can use 100 and 200. New cameras have high iso noise reduction features but you will lose some details…

With more light, use lower ISO. With less light, use higher ISO.

For general sunny conditions, set the ISO to between 200 to 400.
Personally, I leave my ISO at that and rarely alter it. You can adapt to changing light conditions by adjusting the Aperture and/or the Shutter Speed (depending on the type of subject)
If the light gets very low, such as indoors, without a flash, then put the ISO up until you get a decent exposure.

The brighter the sun, the lower the ISO. In sunlight I generally use ISO 100 or if I need a faster shutter speed than your current aperture/iso setting will allow, going up to ISO 200-400. My D600 allows me to go as low as ISO 50, but since that is an "exended" mode, I generally avoid going that far down and stick with the normal settings except for very specific shoots require taking pictures in very bright ligtht and slower shutter speeds are needed.

1) Always select the lowest ISO you can get away with using, but don't go down below base ISO (for most Nikon DSLRs this is ISO 200).

2) High ISOs are more sensitive to light, low ISOs are less sensitive.

a)if you need very high speeds in moderate light, or high speed and lots of DoF you'll need a high ISO.

b) if you high sharpness, use a low ISO. If your shutter speeds are too slow then consider using a tripod. But it won't be able shoot moving objects.

c) in bright conditions a low ISO will still allow you to use moderately fast shutter speeds. This does mean you'll need to use an and filter to get slower shutter speeds.