Defense Against the Dark Arts mainly involved three core themes in the original work.
The first was learning and mastering methods to deal with dark creatures. For instance, in the third year, Lupin walked Harry and the others through a series of dark creatures, such as Boggarts and Red Caps.
The second was understanding dark spells and cultivating ways to counter them to a certain extent. For example, in the original work, Moody (though actually disguised as Barty Crouch Jr.) once used the Imperius Curse on Harry to train his resistance to the spell.
Chapter 44