Rules
Masyu rules: Draw a single, non-intersecting loops passing orthogonally by the center of some cells. The loop must pass by every circled cells. The loop must turn in all black circle, and must go straight in the cells just before and just after. The loop must go straight in all white circle and turn either just before or just after each white circle.
Example
Here is an example of a Masyu grid and its solution
Solve online here
History of Masyu
According to Wikipedia and WPC unofficial wiki, Masyu first appeared in Nikoli in 1999 in a version with only white circles. Then, in 2000 in the version we know today.