An error bar is a means by which uncertainties 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, sometimes with bars along each axis. The bar indicates that the plotted value is only the mean or the most probable value, and the bar's 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 shows as a different length based on the same data, suggesting an indication of the type of error bar helps the viewer. The appropriate choice of type/calculation depends upon the nature of the data and what is being demonstrated.