A real mechanism - terminology and important facts

A real reaction with a known mechanism would be

NO_{2 (g)} + CO_(g) --> NO_(g) + CO_{2 (g)} 

The mechanism by which this reaction occurs is

Step 1:     2 NO_{2 (g)} --> NO_{3 (g)} + NO_(g)

Step 2:     NO_{3 (g)} + CO_(g) --> NO_{2 (g)} + CO_{2 (g)}

Things to note:

• The overall reaction is NOT a description of what is happening at any particular moment. It is, instead, an overall recipe. In other words, when NO₂ and CO are placed together in a container under the right conditions the container will eventually contain NO and CO₂. 
• Each step of the mechanism is called an elementary reaction. This means that the mechanism steps are descriptions of what is actually occurring. So, the first step says that if one NO₂ molecule collides with another NO₂ molecule one of the oxygen atoms may break away from the nitrogen to which it is attached and form a bond with the other nitrogen. 
• The steps do NOT tell you the overall reaction. I would not expect to find a large amount of NO₃ in the container since that is not an overall product. Something like NO₃ (which is produced and then re-used) is called an intermediate. 
• The steps of the mechanism add up to the overall reaction.