Final answer:
The signs in question are called warning signs, marked by their diamond shape and yellow and black colors, save for certain signs like the round railroad crossing signs. They play a crucial role in road safety.
Step-by-step explanation:
The type of sign that is shaped as a diamond and is predominately yellow and black in color, is known as a warning sign. This category of sign is used to alert drivers of potential hazards or changes in road conditions ahead. The exception noted in the question, the railroad crossings warning sign, is indeed round and typically has black lettering on a yellow background.
Such signs are critical for ensuring the safety of motorists and pedestrians alike by providing them with advance notice of road conditions so that they can drive accordingly. Regulatory signs, on the other hand, inform road users of traffic laws and regulations, and stop signs (octagonal and red) are a common example.