ISO sensitivity - basically tells the image sensor in your camera how sensitive you want it to light. The higher you set your ISO the more sensitive it will be towards light (like in low light scenarios, you would 'generally' boost your ISO a little)
BUT, beware, the higher the ISO the more "noise" you
WILL get (in my examples I show you "noise"). I usually shoot in the 200 - 800 range, the lower the
ISO say 200 you absolutely must be on a
TRIPOD, when I'm walking with my camera I usually have it at 400 or 800 (or my camera has auto, if I'm feeling lazy) depending on the light. When I'm taking bud shots I shoot at 200 because I can control the light, and I'm on a
TRIPOD.
My camera is the best right now (well,,, one of them) for lowest noise especially at higher ISO (mine is capable of 6400 ISO, though I will most likely not go there) BUT as you can see noise is still noticeable at 800 ISO
PICS/examples
Not great pictures but they are good examples...
It was about 7:00 p.m. and pretty dark. See in the sky how it's grainy? that's noise - Cool that I could not see the mountains, let alone the snow on 'em
First - I was on this bridge (Sundial Bridge, and you can get a good idea on how dark it was) and it sways making the next 2 pics "not stable" though on a
TRIPOD I was about half way down shooting on the left side
Second - taken at ISO 800 and my shutter stayed open for 30 sec. at f/11
Third - taken at ISO 1600, everything else stayed the same, see the difference in the noise? - I should have shot one at 400 but didn't, my bad.
NOTE: see in the pics towards the 'back' of the pic you see a 'streak'? Those are tail/headlights from cars going over the bridge. I've been on a hunt to get a better example of this....