The difference between a softbox and a diffuser is not drastic since they both are designed to do similar things. Both lighting modifiers create a larger and softer source of light. But they are not the same and you will use each under different circumstances.
It is important to know when to use each in order to achieve the best image quality possible. The needs of your setup will also need to be factored in when picking between a diffuser and a softbox.
When To Use A Softbox
Softboxes are incredibly versatile. They can be used as a primary lighting source as well as filler light.
A softbox allows you to create soft and pleasant lighting for shooting your subjects without harsh shadows. The softness of light can be increased by moving the softbox closer to your subject and when placed properly can achieve the effect of natural light coming through a window.
Softboxes comes in different shapes but usually come in square or octagon shapes. They are constructed from translucent cloth covering a photography light source. Most softboxes are used with a strobe light or speedlight that is mounted on a light stand. The translucent cloth is framed around a wire box and one side is white and the other black.
When the strobe or speedlight is powered on the light is reflected off the cloth and sent in all directions and illuminates your subject in soft, even light.
Softboxes come in many sizes making it possible to achieve the exact lighting you need. They come as small as handheld models all the way up to large wall mounted softboxes. Because of the variety of sizes you can get models ideal for being portable up to dedicated studio lighting options.
Most photographers will have softboxes sized for the type of photography they shoot but it is possible to make any size work as long as you have enough space to accommodate your gear. Because you will have approximately 1 stop of falloff of light on the sides of the softbox, you will most often set your rig about six feet from your subject.
The main difference you will encounter between small and large softboxes is how wide the light will reach without creating shadows. A small softbox can create harsher shadows than larger models so placement becomes more important as the softbox size decreases.
When To Use A Diffuser
Another popular light modifier is the flash diffuser. They are mounted to an external flash on your camera. They also provide soft light by evening out the light when your flash bursts. This helps you create lighting that will make your subjects look significantly better by removing harsh shadows that unmodified light can produce.
One of the most popular versions of diffusers are the snap on type that are constructed of white translucent plastic. This style of diffuser creates what is referred to as the bare bulb effect.
Diffusers come in many different shapes and sizes as well as how much light they diffuse. In general, the bigger your diffuser is the softer it will make your flash lighting.
Because of the way flashes create light, it is often to harsh and direct for creating pleasant images. A diffuser is one of the simplest ways to soften your hotshoe flash since it will spread the light around your subject. This means your scene will be much more evenly lit than it would be with just a flash.
Unlike a softbox though, a diffuser is usually not enough to spread the light over larger scenes. This is one of the biggest differences between a diffuser and a softbox.