Yield signs, as found on roads in the USA, indicate that you are to yield to the stream of traffic on the road you are entering. The standard colors for a yield sign are red and white; red text and white outline, on a white background. According to melknia, they may also be yellow with a black outline and text in Canada.
When approaching an intersection with a yield sign, a driver is to wait and allow traffic in other directions to clear before proceeding. This is somewhat similar to a stop sign, with the exception that you do not need to stop if nobody else is around.
The standard shape for a yield sign is triangular. As far as I know, no others use this shape. Here's the sign in amazing ASCIIvision:
\\ YIELD //