They began as thirty deserters from GAFE (Spanish initials: "Special Forces Airmobile Group") an airborne unit of the Mexican Army, that joined the Gulf Cartel as a hit squad. They grew and got more and more violent, eventually leading to a split from the Gulf Cartel. Because of their aggressive behavior, the Mexican government focused a lot of attention on them, and for much of the early 2010's they were considered Mexico's most dangerous cartel, in part because of their sophistication and resources. In fact in 2012, the Jalisco New Generation Cartel, La Resistencia, and the Gulf Cartel formed a united front against them in Zacatecas, which was partially successful in stopping their expansion.
At their peak they had the largest radio network of any cartel, broadcast stolen equipment and maintained partially by kidnapped technicians. Los Zetas have also made "narco-tanks" also known as "monsters" out of regular vehicles; because of this, more sophisticated weaponry like RPGs stolen from Central American countries, and paramilitary training and structure, some have suggested that they weren't simply a cartel, but "Narco-Terrorists".
Due to low police salaries in Tamaulipas, they basically run the state via bribes and corruption; Nuevo Leon had to reorganize its state police to stop the Zetas influence.
In 2014 following the arrest of its leader, Omar Treviño Morales aka Z-42, the cartel split into two factions, Zetas Vieja Escuela (Spanish: "Old School Z's") and Cartel del Noreste (Spanish:"Northeast Cartel")(the latter of which was led by his brother Juan Francisco. ZVE appears to have joined forces with the Gulf Cartel against the CDN.