Mechanical overview:
Manual sheet metal bending brakes are simple pieces of machinery, consisting of few parts. To make it easier to understand how it works, I will briefly explain the names and roles of these parts:

- The bed is a thick, horizontal piece of steel, whose role it is to support the sheet metal being worked on.

- The clamping leaf is like the upper jaw to the lower jaw of the bed, and is used to clamp the workpiece steadily down to the bed.

- The bending leaf does the actual bending. It is hinged, so that it swings up in front of the bed, thereby bending the clamped-down workpiece. Some of the larger sheet metal brakes have counterweights attached to the bending leaf-levers. This eases the handling, since the bending leaf on a large sheet metal brake can be quite heavy.

- The backgage, on a sheet metal folder, is simply an adjustable stop used to control where on the workpiece the bend takes place.

The bending process:

- The workpiece is laid out on a piece of sheet metal using a premade template and a steel scribe.

- Lines are scribed where bends are going to be performed. Excess material, which would get in the way of bends, is removed.

- The progression of the bends to be made is determined, and the backgage is adjusted for the first bend, so that the scribed bending-line is aligned with outer edge of the brake bed.

- The clamping leaf lever is pulled, clamping the workpiece down.

- The bending leaf is pulled upwards to get the bend to the desired angle.

- The clamping leaf is released, and the process is repeated.

See this video for this exact process:

Now, a regular sheet metal bender is limited in its use, on one specific point: It can only bend in one direction , so to speak.
Case in point: If you want to bend a piece of aluminum u-channel, you could easily do it on a a standard brake.
Simply perform two parallel bends to a sheet of aluminum. However, if you wanted to make a box-shape, IE, a u-channel with the ends bent up to close it into a box or pan, you would run into problems. It would just not be possible to bend the ends up, without crushing
the sides in the process.
Solution:  The box and pan brake.

For more general information about sheet metal brakes: