What’s the difference between Fair Isle knitting, and Intarsia knitting? (I DO know what the duplicate stitch is, but I don’t think it’s the same as either of these.)
Thanks in advance! 
A Question
Have you seen the videos for Color work on the Advanced Techniques page? They should show you what the difference is.
Duplicate stitch; Stranded knitting and intarsia are all forms of using multiple colours on a single row. Double-knitting would be considered by some to be another method, but it’s probably a sub-class of stranded knitting, as is Fair Isle.
In duplicate stitch a needle and colour is used to cover stitches with another colour.
In stranded knitting, two (or sometimes more) colours are carried on the work with one lying behind the fabric and the other being worked, but both being used across the width of the fabric.
Intarsia uses multiple colours, but the yarns work back and forth only through their own sections, interlocking with the other colours to form a fabric.
Fair Isle is a type of stranded knitting which is characterized by certain motifs and patterns of colour shading.
mm
MMario’s description is perfect, and just to add-- what a lot of people call Fair Isle knitting is really stranding. As he said, true Fair Isle knitting also has a bunch of additional rules, such as only 2 colors per row, and uses traditional motifs.
Really helpful and succinct info. I am getting ready to try stranded work and had wondered the same thing.