Tartan is mainly seen as a Scottish fabric used for kilts, originally wool, with a distinctive pattern of squares and lines called a sett. It is known as plaid in North America, whereas in Scotland a plaid is a tartan cloth slung over the shoulder or a blanket.
Before the middle of the nineteenth century the Highland tartans were associated with districts rather than clans, using the natural dyes of that area.
Nowadays the most popular designs are said to be the Royal Stewart and the Black Watch. There are various myths about the significance of the colours used, for example that red tartans were used for battle so that blood stains wouldn't show.
Victoria and Albert popularized all things Scottish in England in the nineteenth century, after buying Balmoral Castle in 1848. It was decorated with a heavy use of tartans and since then the fabric has resurged in fashion on a regular basis; punks, the Bay City Rollers and Burberry, to name a few.
For Clothes Box's latest take on tartan, see this dress by Glamorous, aptly named Highland Fling.
Highland Fling Dress by Glamorous