Most organisms require nitrogen to build amino acids, nucleotides and other biomolecules, but atmospheric nitrogen gas ($N_2$) is chemically very stable and cannot be used directly by most forms of life.

Nitrogen fixation converts $N_2$ into reduced nitrogen, ultimately producing ammonia ($NH_3$) or ammonium ($NH_4^+$), forms that can enter biological metabolism. Biological nitrogen fixation is performed by certain bacteria and archaea using the enzyme complex nitrogenase.

The overall transformation is energetically expensive because breaking the strong nitrogen-nitrogen triple bond requires substantial energy and reducing power.

Plants do not generally fix atmospheric nitrogen themselves. Some obtain fixed nitrogen through symbioses with nitrogen-fixing microorganisms, while others absorb inorganic nitrogen from soil.

Once inorganic nitrogen is available, organisms perform nitrogen assimilation: incorporation of inorganic nitrogen into organic molecules such as amino acids.

The conceptual distinction is therefore:

$$\boxed{N_2 \xrightarrow{\text{fixation}} NH_3/NH_4^+ \xrightarrow{\text{assimilation}} \text{organic nitrogen}.}$$

Fixation introduces previously inaccessible atmospheric nitrogen into biological circulation; assimilation incorporates already-fixed inorganic nitrogen into biomass.