A fine structure is spectral lines resulting from very small differences in energy states of atoms due to differences in electron spin and relativistic effects. When viewed at a low spectral resolution, they may appear as a single line and the term gross structure is used to indicate the spectral lines apparent if you ignore the fine structure. Gross lines can be seen as split into pairs of fine lines, which are smaller in energy level on the order of the square of a constant (the fine structure constant) that is approximately 1/137.
For hydrogen, energy level n=1 is divided by its hyperfine structure, resulting in the 21cm line and energy level n=2 is divided by the fine structure, resulting in a 28.37cm line.
For carbon, the fine structure lines are forbidden lines and are listed as [CI] (490GHz and 810GHz) and [CII] (1.9THz) for neutral and ionized carbon.
Oxygen fine structure lines include 63μm.