Let's start learning about cellular respiration! The first step to it is Glycolysis -- which means "splitting sugar". In this, it takes a glucose molecule, and splits it into two molecules of pyruvic acid. Two ATP are made during this stage!
After Glycolysis, the process of making energy splits into two pathways. The first one is called the aerobic pathway, and it happens when you can breathe properly, or are doing something leisurely.
The second step of the aerobic pathway is called the Kreb's Cycle. What goes in is pyruvic acid, which was formed by glycolysis. What comes out? The electrons (8e-) and hydrogen necessary for the ETC, or Electron Transport Chain.
The electron transport chain (ETC) is a cluster of proteins that transfer electrons through a membrane within mitochondria to form ATP. But what if there's no oxygen for the aerobic pathways? That's called the anaerobic pathway, and there's two types: lactic acid fermentation, and alcoholic fermentation.