iOS 9 brought along a new version of the Notes app with a bunch of new features. However, oddly, if you start using it, you can't sync your notes that use those new features with OS X unless you're on the El Capitan beta. For now, you can use iCloud.com though.
The new Notes app brings along new features and it can't sync with the Notes app on OS X. It's a weird quirk and one that will get worked out when El Capitan releases on 30 September, but for now, you can still view those synced notes from the iCloud webapp.