Tartan is made with alternating bands of coloured (pre-dyed) threads woven as both warp and weft at right angles to each other. The weft is woven in a simple twill, two over—two under the warp, advancing one thread at each pass. This forms visible diagonal lines where different colours cross, which give the appearance of new colours blended from the original ones. The resulting blocks of colour repeat vertically and horizontally in a distinctive pattern of squares and lines known as a sett.
As a child growing up, I used two terms for fabric with a pattern of squares – checks or checkered, and tartan. No one ever explained the difference, and I never thought to ask. Basically, if it was a simple pattern with no overlapping squares, I called them checks, and if the pattern was more complex, with overlapping squares and lines, to me the fabric was tartan.