The Ultimate LUT Guide
EVERYTHING YOU NEED TO KNOW ABOUT LUTS
THE BASICS OF LUTS

WHAT IS A LUT?
HOW DO YOU USE A LUT?
WHAT LUT DO I CHOOSE?
LOG VS. REC.709
There are essentially 2 kinds of footage, Log and Rec.709. Without going into crazy detail, here are the basics...
Rec.709 is the normal-looking footage with good contrast and saturation. Think of it as the kind of images you'd see on TV, not stylized, looks realistic.
Log footage looks really low contrast and de-saturated. Cameras shoot this way to get as much brightness and color information into the recording as possible.
Rec.709 footage looks nice out of the box, but bright things might be too bright, and you might not be able to see darker things. Log footage tries to keep the bright and dark information, but it needs some color work to actually look good.
So there are LUTs that are made to go on Log footage, and some made for Rec.709. A Log LUT will add quite a bit of contrast and saturation to the image before adding style. A Rec.709 LUT will usually add style without any extra contrast or saturation.