D76 is a "fine grain" film developer that comes in powder form. You dissolve the powder in warm water to make a stock solution that can be stored for a few months in closed PETE or glass bottles.
It can be used at stock concentration, or more commonly at 1+1 or 1+3 dilutions.
The fine grain effect is achieved using a silver solvent, sodium sulfite, which dissolves the edges of the film grains. This provides additional smoothness of the final image at the cost of reduced resolution and fine detail. The solvent action is reduced by dilution: D76 stock will look the smoothest, 1+1 will look a little sharper and grainier and 1+3 has very little solvent action at all.
Ilford ID-11 is practically the same formulation as D76 and can be treated in the same way. The same development times apply for these two developers on any given film.
Small white floaty bits that look like shredded tissue can form in D76 stock when stored. Developer in such a state should not be used in stock form because the floaties are highly active and will cause unsightly dense spots on the film (see example image), however dilution to 1+1 will cause them to go back into solution and it will work properly.
D76 should be a clear or very very pale straw colour. If it has gone appreciably yellow (anything approaching apple juice colour), discard it because it may have become inactive.
See general tips on how to store developer.