When two things are “identical” they look exactly alike; there are no differences between them. To the naked eye one seems to be an exact copy of the other.
*The twins were identical. We just couldn’t tell the difference.
When you say that two things are “similar” what you mean is that though they look alike, there are small differences between them. They are not identical. The differences, however, are so minute that they can be overlooked.
*The shirt looks very similar to the one I wore yesterday.