Draw a single loop that passes orthogonally through centers of cells. The loop must go through all cells. The loop may not intersect itself or enter the same cell more than once. The loop must go straight through the cells with white circles. The loop must make a turn in all the cells with black circles.
(Rules and Example from PGP IB)
History of the puzzle
The name of the puzzle suggests that it may have been invented for WPC 1996, but one cannot be too sure about that (see Japanese Sums). Nevertheless, it appeared on 2000 Dutch and German Nationals under the name "Linien ziehen" ("draw lines").  Its author is unknown.