DC Connector

Generic plugs are often described by their inside diameter, such as 2.1 mm DC plugs.
After the two common 5.5 mm OD (Outer Diameter) plugs, the next-most common size is 3.5 mm OD with a 1.3 mm ID (Inner Diameter), usually about 9.5 mm in length, although both longer and shorter versions also exist.
These 3.5 mm OD plugs are normally used for lower voltages and currents.

AC and DC are different from each other, but we use both types of current in our modern society because each has their own advantages and disadvantages, thus making them great for their own applications. AC current is better for the power flowing through our walls because it can be easily scaled up to extremely high voltages without the risk of overheating wires. The voltage running across those lines attached to giant steel towers often exceeds hundreds of thousands of volts, but won't lose too much of that voltage when it's transmitted over extremely long distances.

DC current doesn't do particularly well in this department—scaling it up involves a lot of heat and risk to power lines, and even scaled to high voltages only really has the ability to travel about a mile or two before it loses too much energy. If our homes ran on DC power rather than AC, we'd need to have power plants every couple of blocks or so to service a small number of people. Instead, since we use AC power, we can have one giant power plant that serves thousands of customers, and does so through the use of localized substations.

However, DC current has its advantages as well—it's actually far safer than AC power, and puts you at far lower of an injury risk if you accidentally shock yourself. Likewise, it's far more stable at lower voltage levels, which means it's great for powering our devices. DC circuits are generally far simpler to design, more reliable, produce less heat, and are more stable, which makes them great for electronics or other things which we plug into our walls.

And we're able to compromise and use both through devices known as rectifiers. A rectifier is a device which takes AC current from the wall and converts it into DC. Using other devices known as step-down converters or "buck" converters, we can then turn the higher voltage into low, stable numbers needed to power our devices smoothly and efficiently. In this way, both AC and DC current can actually work together in harmony.