what is the difference between complete and incomplete metamorphosis?

Are you talking about insects?  (you are in earth science and geology section though, so maybe you are talking about metamophosis in rocks...?)
In insects, complete metamorphosis is when insect goes from egg-->larvae-->pupa-->adult.  Larvae look very different from adults (grubs).  These are things like flies, butterflies. beetles, etc.
Incomplete metamorphosis is when an insect goes from egg-->larvae-->adult through successive moltings.  Changes are generally gradual from larvae to adult.  Dragonflies, grasshoppers, etc.
There are also some insects that hatch from egg as miniature adults, and just gets bigger with successive moltings.

Complete Metamorphosis is when an insect larvae looks nothing like it's adult form. It goes through 4 stages of change (or metamorphosis). For example: Butterfly- egg, to larva (caterpillar), pupa (chrysalis), to adult (butterfly).  Incomplete metamorphosis is when an young insect looks like it's adult form, just maybe needs to grow wings and get bigger. For example, a grasshopper is born looking like a small grasshopper (nymph) and has wing pads. As it grows it will get bigger and wings will grow.