Electron transport system means

The metabolic pathway of electron transport is called as electron transport system or ETS.
Glycolysis and Krebs cycle result in the formation of reduced coenzymes such as 10 molecules of NADH ⁺H⁺ ions and 2 molecules of FADH₂ and 4 molecules of ATP. These reduced coenzymes need to be oxidized to release energy stored in them. This is possible by the transport of electrons and protons from these coenzymes to oxygen through electron carriers present in the inner mitochondrial membrane. This metabolic pathway of electron transport is called as electron transport system or ETS. 
  
ETS comprises of several energy carriers which include NADH dehydrogenase complex (Complex I), Ubiquinone (Complex Q), Succinate dehydrogenase complex (complex II), Cytochrome bc1 complex (Complex III), Cytochrome c, Cytochrome c oxidase (Complex IV)

Steps in ETS:
* Electrons are transferred from NADH⁺H⁺ to Ubiquinone through complex I and protons are moved from mitochondrial matrix to intermembrane space.
* Electrons from FADH₂ are transferred to Ubiquinone through complex II and protons are moved from mitochondrial matrix to intermembrane space.
* Ubiquinone transfers the electrons to complex III.
* Complex III transfers electrons to complex IV through Cytochrome c. Some protons are moved from mitochondrial matrix to intermembrane space.
* Complex IV contains Cytochrome a and Cytochrome a³. It transfers the electrons to final acceptor, the oxygen.
* Oxygen combines with 2 H+ ions and reduces to water which drives ETS.
2H⁺ + 2e^- + ½ O₂   --------- H₂O + Energy