The difference between steam and water vapor is that water vapor is typically the same temperature as the air that is present while steam is above the boiling point of water. The chemical composition of water and steam are identical.
The main difference is in how vapor and steam are actually produced and where one or the other exists. Water vapor makes up a large component of Earth's atmosphere and is present as a greenhouse gas and can be found anywhere where water reaches the temperature of the surrounding air. Steam is technically invisible and is created wherever water is found boiling. The boiling point of water is lower at higher altitudes, but is an average of 212 degrees Fahrenheit at lower elevations.