ATP production by direct transfer of a phosphate group from a phosphate-containing molecule to ADP is called substrate level phosphorylation. This process takes place in the cytoplasm during glycolysis or inside the matrix of the mitochondria during the Kreb's cycle, where ATP is formed by physical addition of a phosphate group to ADP. The other type of phosphorylation occurs when ATP is generated from the oxidation of NADH and FADH2 and the subsequent transfer of electrons and pumping of protons.
