Lines can have many different shapes: curved, segmented, straight, and so on. In this article, you can learn about the basic line types available when you first draw your line.
Once you have a line on your page, you can also convert it to a different line without having to redraw it using the Change Line Shape command under the Design tab.
To change the shape of a line, select it (you can also select multiple lines at once by holding down Ctrl and clicking on each in turn) and in the Design tab, choose a new line type in the Change Line Shape menu.
The exception to this is Automatic Connectors used to connect shapes on organizational charts, flowcharts, and mind maps.