The Yellow Smiley is the slang term for the standard minifigure head, having no decoration other than two black dots for eyes and a simple black curve for a smile. From the first use of the modern minifigure in 1978 through the late 1980's, every minifig themed set included only yellow smileys. They were so common that they have become both cliche and classic for many fans who have collected over the years.
Starting with the Pirates theme in 1989, minifig heads began to be available with printed decoration depicting facial hair, eye patches, lipstick, etc. Later themes introduced an increasing variety of non-smiley heads, until the classic smiley became hard to find in new sets in the 21st century.