The reason there are cycles rather than a linear chain of reactions in metabolic processes is because each component of the cycle is a substrate for the subsequent reaction in the cycle. A great example of this is the Krebs cycle.
At the beginning of the cycle, a transfer between a two carbon acetyl group from acetyl-CoA (acetyl coenzyme A) and oxaloacetate form citrate. Then, throughout the cycle, the citrate reacts with different enzymes and releases water and CO2 as it goes, and also picks up/releases electrons from a compound nicknamed NADH or NAD (depending upon if it is oxidized or reduced with a hydrogen ion). Finally, the substrate fumerate reacts with the enzyme fumerase to form the product malate (by means of a hydration reaction). The malate combines with NAD and the enzyme malate dehydrogenase to form oxaloacetate, which brings us to the end of the first cycle of the Krebs cycle. As you know, the oxaloacetate produced at the end of the cycle will react with the acetyl group in acetyl-CoA to form citrate. Thus, the start of the second cycle of the Krebs cycle.
The reason there are 2 cycles in this metabolic process is because two acetyl-CoA molecules are produced from a each glucose molecule formed in glycolysis (which is a preceding metabolic process to the Krebs cycle). Since 2 glucose molecules are produced from glycolysis, two cycles are required per glucose molecule.
So, in conclusion, the Krebs cycle starts with oxaloacetate and acetyl-CoA to form citrate. The citrate is reacted with other enzymes to form other compounds that eventually brings us to the formation of malate, which reacts with the enzyme malate dehydrogenase to form oxaloacetate, which will react with the other molecule of acetyl-CoA, thus beginning the second turn of the Krebs cycle.
Hopefully I didn't confuse you =]