"Plata o plomo" ("silver or lead"), is an expression meaning bribery or violence. Mexico's cartels have coalesced around the two most powerful, the Sinaloa and the Los Zetas, who tend to operate at opposite ends of that spectrum. They use the same set of tools, but they use them to different degrees.
The Sinaloa leaders have been in "business" for decades, tend to take a more patient approach to things, and generally aim to own officials and politicians. Not that they aren't capable of ruthless brutality, it's just not usually their preferred Plan A. Like intelligence organizations, they often bribe low-level officials and then facilitate their promotions. Sinaloa has formed very long-term relationships with many police and military officers, politicians, journalists and judges on both sides of the border, paying some of them millions of dollars per year.
Sinaloa head Joaquin "El Chapo" Guzman is an almost mythical figure who has been a fugitive for over thirty years. Having bought a great deal of goodwill, he is revered, even, among Mexico's poor. He was arrested in 1993, continued to run his empire from a very comfortable prison situation, and "escaped" when it looked like he might be extradited to the US. Since then, both Mexico and the US have been "unable" to locate him.
The Los Zetas cartel was more recently formed by Mexican special operations soldiers, who have stolen and bought military weapons and know how to use them. It tends to apply a violent solution to any problem first. They bribe people, but find that it is faster, cheaper and easier to intimidate or eliminate them. Even those whom they choose to bribe tend not to stay on their payroll for years.
As a general rule, when you think 'corruption', think Sinaloa. When you think 'piles of heads dumped in a public square', think Los Zetas.
Mexican officials are under a great deal of pressure to "do something" as their presidential election approaches, and US officials as the "Fast and Furious" scandal gets ever dirtier. Stay tuned for exciting developments.