Thousand Island dressing, pink in color, is a variety of salad dressing, a variant of Russian dressing, commonly made of mayonnaise, ketchup, and a mixture of finely chopped vegetables, most often pickles, onions, bell peppers, and/or green olives; chopped hard-boiled egg is also common.
   It is used both in salads and as a sauce on sandwiches, especially in fast-food restaurants.

It tastes (and appears to be) very similar to Burger sauce, which is a 'simpler' version of thousand island dressing and is commonly used in fast food outlets in the UK. Burger sauce is made out of similar ingredients, such as tomato ketchup, mustard and mayonnaise.
   In many areas of Europe, what is marketed as "American Dressing" is actually Thousand Island Dressing.

Origins

1. It was invented at Chicago's Blackstone Hotel in 1910.(External Link)
2. Sophia LaLonde invented it in the first decades of the 20th century, substituting mayonnaise for the yogurt used in Russian dressing, and added pickle relish, chives and, sometimes, chopped, hard-boiled eggs. The dressing was popularized by one of her dinner guests, actress May Irwin, who gave the condiment its name, after LaLonde's home, the Thousand Islands region of upstate New York and Eastern Ontario.
3. The name refers to the multitude of small specks of pickle usually found in the dressing.
4. George Boldt, of Waldorf-Astoria Hotel fame, popularized it by instructing his maitre d'hotel, Oscar Tschirky, to put the dressing on the hotel's menu. Boldt had a home called Boldt Castle on one of the Thousand Islands.

Uses

Around half of the selections on the menu at Pizza Hut in Hong Kong use a Thousand Island dressing for the base, instead of traditional tomato sauce. (External Link)