In a Non polar Covalent Bond, atoms share their electrons equally. That happens when, they have the same electronegativity or the same electron affinity. The best examples for this kind of bonds are molecules created through the union of atoms of the same element, as in H2, N2, O2, Cl2
When non-polar molecules interact among them, there are negligible forces of attraction, and they remain as individual molecules, that means these substances are gases.
In our environment we have these substances in the Air, which has approximately the following composition, 78% of N2, 21% of O2, 0.9% Ar (noble gas)