A softbox is a type lighting equipment. It is a lightweight portable light & light difusser all built into one. They are used for film & photography. 

A light diffuser provides even, soft light onto the subject being photographed. They are used heavily in portrait photography and many other types of commercial photography.

Softboxes are a popular choice for a light difuser for photographers as they are lightweight comonly made wtih a synthetic fabric housing that can also be collapsed for storage or transport. Another benefit is that they are relativedly easy to mount up high on a light weight stand.

Softboxes vary in size alot, commonly a photography may use one that is merely 28 x 21 inches or so, in film production though, softboxes can be several feet across and hoisted above the cast & crew via an industrial crane.

Choosing a Softbox Light:
The greatest consideration when choosing a softbox for light diffusion is where and what you will be using the softbox for. For example if you need to transport the softbox frequently perhaps a higher quality one will serve you better, one that is durable and very lightweight. If you are working almost exclusively in a studio setting then a cheaper softbox that is perhaps heavier may surfice. Size wise, depends on the size of the subject matter you are filming or photographing and how much diffused light you require. 

Where to buy a Softbox Light
Photography lighting equipment can be purchased from any good Photography Supples Store, alternatively you can buy a softbox and many other varied types of studio lights, or hire studuo lighting from a specialist Lighting Equipment Supplier.