Off the top of my head, the ICD is the International version of the DSM, its basically another book that is used to classify and diagnose mental disorders, but is is different to the DSM in some ways
both the ICD and DSM use a categorical approach, yet i think one of a few differences is that the DSM classifies mental and behavioral disorders as the same thing and the ICD classifies them separately. Their are more differences between the two though that you should read up on.
Differences between an individual with mental health and a mental illness is that an individual that is mentally healthy can function effectively whereas one with mental illness cannot, their life is impeded by their debilitating condition, furthermore an individual that has a mental illness struggles to maintain relationships with others while mentally healthy does not.
Probably not the best summary but i hope it helps.