In general, a catalyst is an agent that increases the speed of a chemical reaction without being consumed by a reaction. Thermodynamically, a catalyst lowers the activation energy required for a chemical reaction to take place. An electrocatalyst is a catalyst that affects the activation energy of an electrochemical reaction. Shown below is the activation energy of chemical reactions as it relates to the energies of products and reactants. The activation energy in electrochemical processes is related to the potential, i.e. voltage, at which a reaction occurs. Thus, electrocatalysts frequently change the potential at which oxidation and reduction processes are observed. Alternatively, an electrocatalyst can be thought of as an agent that facilitates a specific chemical interaction at an electrode surface. Given that electrochemical reactions occur when electrons are passed from one chemical species to another, favorable interactions at an electrode surface increase the likelihood of electrochemical transformations occurring, thus reducing the potential required to achieve these transformations.
Electrocatalysts can be evaluated according to three figures of merit: activity, stability, and selectivity. The activity of electrocatalysts can be assessed quantitatively by understanding how much current density is generated, and therefore how fast a reaction is taking place, for a given applied potential. This relationship is described with the Tafel equation. In assessing the stability of electrocatalysts, the ability of catalysts to withstand the potentials at which transformations are occurring is crucial. The selectivity of electrocatalysts refers to their preferential interaction with particular substrates, and their generation of a single product. Selectivity can be quantitatively assessed through a selectivity coefficient, which compares the response of the material to the desired analyte or substrate with the response to other interferents.
In many electrochemical systems, including galvanic cells, fuel cells and various forms of electrolytic cells, a drawback is that they can suffer from high activation barriers.  The energy diverted to overcome these activation barriers is transformed into heat.  In most exothermic combustion reactions this heat would simply propagate the reaction catalytically.  In a redox reaction, this heat is a useless byproduct lost to the system.  The extra energy required to overcome kinetic barriers is usually described in terms of low faradaic efficiency and high overpotentials.  In these systems, each of the two electrodes and its associated half-cell would require its own specialized electrocatalyst.
Half-reactions involving multiple steps, multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers.  Furthermore, there is often more than one possible reaction at the surface of an electrode.  For example, during the electrolysis of water, the anode can oxidize water through a two electron process to hydrogen peroxide or a four electron process to oxygen.  The presence of an electrocatalyst could facilitate either of the reaction pathways.