iTunes music sync clarification
Sync only checked songs and videos

When you open the list view, the leftmost column is of check marks. You can use these to tell iTunes what to sync/not sync on a per song basis.

Selected playlists, artists, albums and genres

When you open the summary for your iDevice, there is a Music tab. You can select what songs to sync on a per playlist/artist/album/genre basis.

Manually manage music and videos

This means that iTunes will not sync all songs indiscriminately. You have to manually drag songs to the device to sync them. You can also drag albums and playlists.