Yin and Yang are represented symbolically by a broken line and a single solid line respectively.
If you take two Yang Lines, place it at the bottom and then add a Yang Line and a Yin line on top, you get a combination known as Greater Yang and Lesser Yin. Do the same with the Yin Lines and you get another two combination known as Greater Yin and Lesser Yang as shown below.
Finally if you add a Yin or a Yang line to each of the symbols generated above you get eight different symbols each known as a Kua.
Gua or Kua refers to the same thing. This is due to translation from the different Chinese dialect resulting in different pronunciation and spelling. A Kua is also frequently called Trigram.
Notice that there are eight Kuas. Collectively they are known as Pa Kua or Eight Gua. It is also written in many text as Ba Gua which means the same thing. In English it is known as the Eight Trigram. Each kua is given a name. They are Qian, Dui, Li, Zhen, Xun, Kan, Ken and Kun.
Historian belief that the Kuas are symbolic representation since written text most likely did not exist then. But what do they represent? More the Pa Kua here.
(Feng Shui Kua Trivial: Kua or Gua, the same)