Emiliano Zapata was born a mestizo (of mixed Indian/Spanish blood) in the small town of Anenecuilco, Morelos in 1879, just as President Porfirio Díaz was consolidating his power. He was born into a poor farming family, but they were fortunate in that they owned their land. Many of the people of Morelos and in other southern states worked as debt peons on the sugar plantations. The debt peonage system was only one step up from slavery: the workers were dependent on the plantation for all of their food and shelter and often owed the "company store" more than they could ever work off. Conditions were unspeakably harsh: peons were routinely beaten and if they tried to run off they were hunted down and killed.
As a young man, Zapata was known in his native Morelos as an extremely talented horseman. He cut a dashing figure in his gaudy charro (a charro is a sort of Mexican Cowboy) clothes, riding horses and fighting bulls in the annual town fairs and rodeos. His fame as a horseman grew enough that he was offered positions as horse trainer on prestigious plantations: under other circumstances, he might have lived out his life as a relatively well-paid horse trainer.
In 1908 he seduced Inés Alfaro, a woman from Cuautla, and brought her back to Anenecuilco, where they lived together and quickly had three children. It was a bit of a scandal and resulted in his being drafted into the army: he served for a while and then bought his way out; he was back home by 1909. Zapata loved women and was known as a ladies' man: during the Mexican Revolution he would go through dozens of lovers.
Like many poor farmers, Zapata suffered greatly during the Porfiriato, the time in which President Porfirio Díaz ruled Mexico with an iron fist (1876-1911). There was conflict in Morelos during this time: traditional farmers and villages against enormous sugarcane plantations. Sugarcane had proven very lucrative, and President Díaz allowed (some would say encouraged) his wealthy cronies to blatantly steal any land they felt they needed to grow sugar. They often took lands which had belonged to local villages for centuries. This was made easier by the fact that the villages rarely had any sort of paperwork proving ownership.
In 1909, charismatic young Zapata was elected mayor of his home town of Anenecuilco. He immediately began fighting for the return of land stolen by nearby sugarcane plantations, and initially had some success, as Anenecuilco actually did have some ancient papers proving ownership of the lands in question. The papers, some of which dated back to the colonial era and many of which were written not in Spanish but in Nahuatl, were remarkably complete, proving without a doubt that the plantations had been stealing private and community land. Zapata had the papers translated and buried in a tin box for protection.
When Zapata confronted the wealthy plantation owners about their theft of village land, they predictably told him to get lost. He took his case to the state governor, a crony of President Porfirio Díaz. The state governor stalled, however, and Zapata was forced into action. In mid-1910 he armed a small force of disgruntled peasants and began occupying the sugarcane plantations, forcefully giving back the illegally obtained land.
Had Zapata been so bold in, say, 1905, Díaz simply would have sent an army to Morelos and hunted down the rebels like dogs and slaughtered them. The situation in 1910, however, was different. Díaz had a big problem on his hands in the form of Francisco I. Madero, who had run against him in the 1910 election. When it became clear that Madero would win, Díaz trumped up some charges and threw Madero in prison. Bailed out by his wealthy family, Madero went to the United States, proclaimed himself rightful president of Mexico, and called for armed revolt.
Zapata saw this as an opportunity, quickly joining Madero and his Revolution. This, he thought, is a true chance for land reform in Mexico.Source: McLynn, Frank. Villa and Zapata: A History of the Mexican Revolution. New York: Carroll and Graf, 2000.