What is a constellation? It is simply a collection of stars, which in ancient times were related to mythical figures such as gods and legendary heroes. I do not know how the ancient Greeks came to find shapes to the constellations (In these modern times with three tequilas is enough). It is necessary to consider that in the antiquity they had the great advantage, as far as the astronomical speaking, that there was not the light pollution of artificial light reason why they saw much more stars of even tenuous brightness, perhaps for that reason they found more "form" to the figure.

These two little v-shaped lines are fish, according to the ancients.

The constellations help us to delimit the sky in well-defined areas, in short, to create a map. This makes it easier to locate a particular deep-space object, planet or star.

Amateur astronomers today use a mixture of ancient and modern constellations (such as the constellation "the microscope").

Star names and denominations
There are bright stars that have been called by various names by different civilizations. The names that have survived until the present day are mainly due to the Arabs and Greeks. Some examples of the most popular stars are Sirius in the constellation of Can Mayor, Polaris or the polar star in the constellation of Ursa Minor, or Alnilam, Alnitak and Mintaka which are the three stars that form Orion's belt (also called the "3 kings").

Let's think about this: can you imagine finding a proper name for each of the stars in the sky ? We would never finish and it would be difficult to find a different name for each of them.

That is why there are systems of naming stars, which make it easier for us to name and identify them. One of the most popular and used among amateur astronomers was the one created by the German astronomer Johannes Bayer, and it consists of the following: it classifies the stars of a constellation from the brightest to the dimmest using the name of the letters of the Greek alphabet.

For example, the brightest is called alpha, followed by beta, then gamma, and so on. Then it adds the Latin genitive according to the constellation it belongs to. For example, the brightest star in the constellation of Sagittarius would be Alpha Sagitarii, the second brightest star in the constellation of Orion would be Beta Orioni, etc. In this way it is easier to name and refer to the stars of a constellation.

Let's do the following exercise: 

This zone in the sky is called "Rho Ofiuco area". Let's look at the map of the constellation of Ophiuchus.

If we look at the Greek alphabet reference, the letter rho is ρ ( which resembles a pe ). In conclusion, the area of the inside where he pointed his camera to take the picture was the following:

Asterisms
Nothing to do with asterisks, don't be confused. A section or figure of a constellation that is easily identifiable is called an asterism. We must remember that the constellation is formed by many stars but many times, given the quality of the sky, we will not be able to see them all with the naked eye; however, the brightest ones are those that stand out, showing us only a part of the constellation.

The most famous and classic are the "dipper" of the Ursa Major and Ursa Minor constellations. These asterisms are commonly identified as if they were the complete constellation but no, they are not. They are only those that are always observed with the naked eye.

References
Wikipedia: Stars list
Bayer designation