Generally they reduce the coefficient of friction of the wearing surfaces (or clutch plates in a transmission), not the lubricant. Some common ones are boron and moly which will plate out on surfaces. I have also seen organic esters used as friction modifiers/anti-wear additives, since they tend to adhere to metallic substrates.
Our member Molakule wrote this in another thread.I bet he could ad much more. It I think is a very good read,,Molakule hope you don't mind me pasting this here!
EP or extreme pressure additives are generally accepted as those additives used in gear and transmission oils because of the extreme pressures exerted on gear teeth. (See the thread on interesting articles. regarding TBN#'s). EP's are generally orgnometallic films of elements that include phosphorus, sulfur, and chlorine. Other organometallic compounds may include moly, antimony, and zinc, and certain esters, or all of the above.
Anti-wear (AW) and Friction Modifiers (FM's) additives are generally accepted as terms used for additives in motor oils, to reduce boundary friction. When the hydrodynamic lubrication is present (wedge of oil produced by eccentricities of the bore/journal) the journal and bearing are separated by a thin film of oil. If a high load is applied suddenly, or if the temp rises suddenly, the film may disappear momentarily, and this is when boundary lubrication saves the day. Boundary lubrication most often occurs in camshaft assemblies and the oil control ring of pistons. If your oil pressure were to suddenly drop, and the hydrodynamic film in the journal bearing were to be lost, boundary lubrication is the only "last ditch" effort to save the engine.
Now, ZDDP can be found as an additive for both EP and AW/FM applications, as well as organometallic compounds including moly, antimony, and zinc, and esters. That's why these compounds are often called, "multifunctional" fluids since they may have more than one function.