"what is the key difference between expressed and implied powers"

An expressed power is one that is specifically written down in the constitution. For example, the power to collect taxes, borrow money, regulate commerce, and establish a post office and post roads are all implied powers-they are stated specifically in the constitution.
Implied powers are those that are not stated specifically in the constitution, but are implied by the "necessary and proper clause" in the constituion. This clause gives congress the right to pass any laws that they think are necessary in order to enforce or carry out the expressed powers. One example is that, becuase it has the right to coin money, congress also has the right to set up a national bank.