It is named after its "rhombic" diamond shape, with each side typically being of wavelength size. Each vertex is supported by a pole, several meters tall. It is fed at one of the sharp angles through a balun transformer, and is terminated at the opposite sharp angle with a non-inductive resistor. It is directional toward the resistor end, so it "points" toward the region of the world it is designed to serve.
The rhombic antenna can radiate close to the horizon or at a higher angle depending on its length, width, and height relative to the operating frequency. Likewise, its beam can be narrow or broad. A proper combination of size, height, and operating frequency make it fit for short- medium- or long range communication.
Due to its considerable size, it is not very practical as the sole antenna of a radio station if operating conditions are expected to change rapidly. Moreover, it plainly requires a lot of land - especially if several must be combined to serve a variety of georgaphic targest at different distance ranges and on widely different frequencies.
On the other hand, it is one of the best options for predictable point-to-point circuits. Its very size give it a degree of gain, and allow it to capture energy from a wide area, thus making it a little less susceptible to sharply localized fading than smaller antennas.