An error bar is a means by which variations in observations or calculations can be expressed. It consists of a line segment crossing a point or the top of a bar graph showing a range of values around the indicated value. Similar bars are used for plots of points. The bar indicates that the point is only the mean or the most probable value, and its length indicates some statistic regarding distribution of either the measured values, the sample size, or sources of error. Commonly used statistics for the bar:
A fourth usage of such bars, probably best not described as "error bars", simply indicates the full range of data, i.e., marking the highest and lowest value.
As a general rule, the shorter the error bar, the "better" or "more significant", but each of the three types has its own formula and produces a different length based on the same data, suggesting the type of error bar used should always be cited. The appropriate choice of type/calculation depends upon the nature of the data and what is being demonstrated.