DC connectors are typically used for power supply of low voltage equipment below 50V, 1-2A.
It is distinguished by the round barrel shape, with a center hole. One pole is connected to the outer part of the barrell and the other pole is connected to the center hole.
This connector today comes in a variety of sizes, where the center hole diameter and barrel outer diameter is specified, e.g 2.1mm and 5.5mm.