The Olmecs were the first known Mesoamerican civilization, and one of the first known in the Americas as a whole (only preceded by the Norte Chico civilization, which is considered to be the cradle of civilization for the Americas). Olmecs inhabited the tropical lowlands of the modern-day Mexican states of Veracruz and Tabasco (central eastern Mexico), with the most prominent cities being in San Lorenzo, La Venta, Tres Zapotes, and Laguna de los Cerros. The civilization formed around 1500 BCE and lasted until around 400 BCE. The Olmecs are known mostly for their artwork, particularly the colossal heads, found all over their territories.
This civilization emerged from the agricultural societies which had formed in and around the Coatzacoalcos river basin. While the river is not as large, it and the environment of the area can be compared to the Indus, Yellow River, Nile, and Tigris and Euphrates valleys, which means that even in the Americas, the same pattern of the first civilizations forming around major rivers followed. Anyways, at their height the Olmecs not only sculpted those huge heads, but they also crafted statues, figurines, drinking vessels, mosaics, face masks, votive axes (little statues used for rituals), and other luxury artifacts from materials such as jade, basalt, greenstone, obsidian, and magnetite, which came from distant locations. The early Olmec elites had access to an extensive trading network in Mesoamerica and so managed to acquire a lot of these materials by trading them for their own produce, such as food, of which Olmecs must have had more than enough for themselves.
The Olmecs are thought to have been the first civilization to engage in bloodletting and human sacrifice, which is usually attributed to Aztecs, but now you'll know where they got it from. They also might have very well been the first civilization in Americas to use writing, with their symbols dating at least to 650 BCE and potentially up to 1100 BCE. Their script is similar to the Mayan one, so again, they would be the progenitors here. Other inventions include the Mesoamerican Long Count calendar (used by many subsequent Mesoamerican civilizations), and the Mesoamerican ballgame, an important religious and recreational activity in the region in pre-Columbian times. As for political organization - there were influential priests, but the main leaders were the kings. It is thought that it was a rather centralized society, so it could have been a kingdom ruled directly from its capital, but it's hard to say for far it stretched and whether it had much influence over the countryside or was mostly confined to the few major cities.
While their first capital was in San Lorenzo, it was abandoned around 900 BCE due to environmental changes, so La Venta became the most prominent site afterwards. However, after 400 BCE this too was abandoned, along with most other Olmec sites, so this is cited as the end date for the civilization. There were no outside conquerors, so natural disasters, such as tectonic upheavals or volcanic eruptions, are cited as the main reason for the collapse of it. Tres Zapotes in the west was still inhabited but wouldn't continue to bear the marks of the Olmec civilization and most probably already shifted to the post-Olmec (known as Epi-Olmec) culture.
