What is the difference between the middle passage and the triangular trade?

The Middle Passage specific refers to the journey the Africans slaves made from Africa to the Americas. It is called that because it was 2nd trip made by European slaves ships during the Triangular Trade.

The Triangular trade refers to the entire process, and similar to what you described in your question. That is, the European ships traveled to Africa to sell manufactured products for slaves; then these ships took the slaves and continued on across the Atlantic to the Americas (the middle passage), and traded the slaves for raw materials (like cotton) from the colonies; and finally they took these raw materials back to Europe to make clothing and whatever.