I am curious to know what are dead cells are they like germs? Do they cause diseases

Here's a comprehensive site: Your Skin

Cells in your body die everyday, your skin being one of the types of cells as described by the link of Judy.

To make you think more about it... vaccines are an example where people inject dead/no more harmful viruses in your body. If they were harmful I don't think that doctors would do that right? :)

Well, dead cells are simply dead, that is they don't do anything and are usually quickly removed from the body, or used for another purpose, like your dead blood cells. You will probably later learn that they are used in bile, a substance that helps digestion when the cells died.

They die and we shed them. They are then replaced. Millions and millions of cells...