Final answer:
Data Bars in Microsoft Excel are used for conditional formatting to graphically represent the relative size of values in a range with horizontal bars directly within the cells.
Step-by-step explanation:
The type of conditional formatting in Microsoft Excel that displays horizontal colors where the width of the bar indicates relative size compared to other values in the selected range is called Data Bars. This feature visually helps to identify larger and smaller numbers by the length of the bars within the cells, offering a quick graphical representation of the data. Unlike Color Scales that shade cells with different colors based on their values or Icon Sets that assign icons, Data Bars fill the cell with a horizontal bar, providing an immediate visual cue about value length and distribution.