Many iMac models, including the iMac (Summer 2000), iMac (Early 2001), and the iMac (Summer 2001) have a VGA port which provides the capability to mirror the image displayed on the internal display to an external display. The external display must utilize a VGA style connector to match that on the iMac. Refer to figure 1 for an image of a VGA connector.
Figure 1 VGA connector