In musical notation, a bar (or measure) is a segment of time defined as a given number of beats of a given duration. Typically, a piece consists of several bars of the same length, and in modern musical notation the number of beats in each bar is specified at the beginning of the score by a time signature (such as 3/4).
http://en.wikipedia.org/wiki/Bar_%28music%29 ^^
it talks about bar lines also