A tablet is a pressure sensitve writing pad.
With it, comes a pen which is used instead of the conventional mouse.

Easier to use
The digital pen is held and used like any other writing implement and the use of it comes more naturally to us than the mouse does.
More sensitive
A digital pen is extremely sensitive and is capable of registering very fine detail in a precise manner. Smooth, effortless strokes are considerably easier to achieve with a digital pen than with a mouse. A pen is pressure sensitive and can give at least 512 levels of pressure, enabling complete control when making strokes lighter or darker and larger and smaller. It is the digital equivalent of the traditional airbrush.
A mouse cannot do any of this.
More controllable
When doing photographic retouching and digital painting, brush sizes and densities can be changed in a continuous flowing manner as you paint. While working on very small areas, this function is a vast bonus.
Using a mouse, brush size, brush softness and brush size have to be change continually in the Brushes panel. With the pen, this can be accomplished seamlessly as you are making the strokes.

More versatile
Many of Photoshop’s brush tools work with a pen and extra options become available. Tools such as the Brush, Pencil, Eraser and Smudge tool all have greater verstility with the digital pen.
No RSI
It is not uncommon for long-term users of the mouse to suffer from repetitive strain injury.
A pen, with its easier and more natural holding position, avoids this.

Market leaders in the world of digital pad and pen are the Wacom Graphire, the standard version, and the Wacom Intuos. The Intuos is the professional version; it has a larger range of sizes, more pressure levels, higher pen resolution, a choice of pen nibs and shortcut keys on the pad. The device can be attached to the computer via USB or for complete freedom, there is the wireless version using Bluetooth.
One tip
Stick to the 6" x 8" size. For most people, anything bigger is unnecessary and can be a nuisance.

www.the-graphics-tablet.com goes into much greater detail of the benefits and features of this useful gadget.
There are also many tips about setting up and using the system.