Now this one has a bigger bite to it than the rest, but is still quite straightforward. You have a pre-existing cantus firmus, aka given melody. And then, you have five different ways to add a new voice to your given melody. These are the five species of counterpoint. Each species has a characteristic.
First species: The added voice(s) move(s) along with the cantus firmus. One against one. All voices share the same note durations. If the voice of the bass is in half notes, the voice of the soprano will be in half notes as well. If the soprano is in eighth notes, the alto will also be in eighth notes.
Second species: It’s one against two. The one note of the cantus firmus corresponds to two notes of the added voice. If the given voice is in half notes, the added voice will be in quarter notes. Two quarter notes correspond to one half note.
Third species: One against four. Usually, the given voice is in whole notes and the added voice is in quarter notes.
Fourth species: One against one… ish. Think of the first species with a twist. The added voice is shifted by half the duration of the given voice’s notes. The notes of the added voice are heard before and after the notes of the given voice, creating suspensions and thus forming something like a dialogue between the voices.
Fifth species: All of the above mixed together. It’s a free-for-all. It’s a funfest…