shareholder capital is the value of the shares of stock you own at par value
additional paid in shareholder capital is the value you paid over and above par value
As I understand things, a coproration is a legal entity, it stands alone legally. Whereas a sole proprietor or partnership is you personally being liable legally. (LLC adds personal liability protection much like the protection that a corporation does, but that is at the state level)
So just like your loaning money to a friend, that is a legal loan, you do not become part of your friend. He pays you back eventually, and if not then you have civil recourse to sue him. The corporation acts as the same separate legal entity.
This may help