Hechsher

A hechsher ( "prior approval"; plural: "hechsherim") is a symbol placed on food bought at a grocery store that is considered kosher by a professional kosher checker.
There are multiple hechschers given by different companies. One popular company is the OU, which has a symbol of a "U" with a surrounding circle. Another popular company is the OK, which has a symbol of a "K" with a surrounding circle.