Soft boxes are called soft boxes because they softens the light source, the strobe is placed inside the soft box, or more correctly - the soft box is mounted on the strobe head which again is to be mounted on a stand.
Soft boxes can be found in several shapes and sizes.
They come in various formats: F.x.70 x 100cm, 80 x 120 cm and 60 x 60 cm. Also, you can find soft boxes that are strips. A size can be like 22x90cm, 25x140cm or 30x180 cm. Very useful for portraits in full figure.
For both kinds you can buy them with a honeycomb to be mounted on the front of the soft box. This works the way that it will not spread the light as much, but will narrow the beam of light onto the surface.
Soft boxes are available in rectangular, square, octagons or strips.
Some octagons are sized; 60, 90, 120, 150 or 170 cm.
Soft boxes can be troublesome to put together. Normally, there are metal rods that should be mounted in each corner of the box and then to be attached to a ring, that will be the ring that has to be mounted to the strobe head. This fitting could be a Bowens mount, but others are available of course.
Inside the soft box, is a piece of fabric placed in front of the flash tube, this helps to spread out the light, and make it bounce around, before being let through the front fabric. The inner fabric is white translucent, and does not reach out in the corners of the box. This will again let some light easier reach the front white fabric, and all the light will spread more evenly, also out to the corners.
The inside of the box is of some silver looking material, or plastic coating that helps to bounce the light around and will reflect more light in the box - to be let out, through the front cover.
Above is a photo of a soft box, without the front cover. You can see the sheet of fabric inside, attached to the corners of the soft box, inside.
And the other photo is a close up of the material inside, looking like silver droplets.