1NF is the most basic of normal forms - each cell in a table must contain only one piece of information, and there can be no duplicate rows.
2NF and 3NF are all about being dependent on the primary key. Recall that a primary key can be made up of multiple columns. As Chris said in his response:
The data depends on the key [1NF], the whole key [2NF] and nothing but the key [3NF] (so help me [Codd][1]).
Say you have a table containing courses that are taken in a certain semester, and you have the following data: