The difference between composite and weak entities?
A Composite entity can be made out of two Primary Keys (PK) of two different entities. Therefore it can be called as a link table. Therefore the composite entity depends upon the other two entites. Therefore the composite entity is a exsitence dependent on the other two entities.
This reminds me of one of my favorite sayings: In theory, theory and practice are identical. In practice, theory and practice are unrelated.
In all normal cases, a composite entity must be a weak entity because it depends on the two entities that it links.
There are a few cases (all the ones that I can think of are governmental) that can be exceptions to this rule. There is some debate as to whether or not these can even be modeled as weak entities.
One example would be a Louisiana tax purchase, where the taxes have not been paid on a property for too long and the property is put on the tax auction. Because of the way that Louisiana law works, the "purchase" is presumed and actually almost completely processed before the auction occurs, so the buyer is unknown at that point. Once the sale occurs, then the property identifier, the buyer, and the date become a natural candidate key, but they aren't even a candidate key until then because the buyer is unknown.
For all of the cases that a data modeling class would cover, a composite entity is a weak entity. In practice, there are (arguably) a tiny number of exceptions.