USB Type-C (or USB-C)

USB 3.1: Released in July 26, 2013, USB 3.1 doubles the speed of USB 3.0 to 10Gbps (now called SuperSpeed+ or SuperSpeed USB 10 Gbps), making it as fast as the original Thunderbolt standard. USB 3.1 is backward-compatible with USB 3.0 and USB 2.0. USB 3.1 has three power profiles (according to USB Power Delivery Specification), and allows larger devices to draw power from a host: up to 2A at 5V (for a power consumption of up to 10W), and optionally up to 5A at either 12V (60W) or 20V (100W). The first USB 3.1 products are expected to be available next year, and will mostly use USB Type-C design.

USB Type-C (or USB-C)

Physically, the Type-C port and connector is about the same size as that of the Micro-B USB mentioned above. A Type-C port measure just 8.4mm by 2.6mm. This means it's small enough to work for even the smallest peripheral devices. With Type-C, a USB cable's both ends will be the same, allowing for reversible plug orientation. You also don't need to worry about plugging it in upside down.

Set to be widely available starting 2015, Type-C USB will support USB 3.1 with the top speed of 10Gbps and has much high power output of up to 20V(100W) and 5A. Considering most 15-inch notebook computers require just around 60W of power, this means in the future laptop computers can be charged the way tablets and smartphones are now, via their little USB port. In fact Apple's latest 12-inch Macbook is the first notebook that incorporates a Type-C USB as its power port.

Going forwards, USB-C will enable storage vendors to make bus-powered (no separate power adapter required) external hard drives of much larger capacity, since it provides enough power to run one or even multiple desktop hard drives.

Type-C USB also allows for bi-directional power, so apart from charging the peripheral device, when applicable, a peripheral device could also charge a host device. All this means you can do away with an array of proprietary power adapters and USB cables, and move to a single robust and tiny solution that works for all devices. Type-C USB will significantly cut down the a amount of wires currently needed to make devices work.

Backward-compatible but adapters required
Type-C USB and USB 3.1 are backward-compatible with USB 3.0 and USB 2.0. In a pure Type-C USB connection, the Type-A ports and plugs are no longer included. However, for compatibility, you'll find compatible Type A to Type C cable. On top of that there will be adapters to make Type C hosts and devices work with existing USB devices.
This is the first time adapters are required with USB, and likely the only time, at least for the the foreseeable future. USB Implementers Forum, the group responsible for the development of USB, says that Type-C USB is designed to be future-proof, meaning the design will be used for future and faster USB versions.
It will take a few years for Type-C become as popular as the current Type-A, but when it does it will simplify the way we work with devices. There will be just one tiny cable needed for any device, for both data and power connections.