Yes! You can add notes for each client or project easily.
When viewing a client's details page, navigate to the Notes section and click "Edit" to add them. These notes are private and only visible to your company (your account and any partners you have added to help manage your company).
You can add project note right from the project's overview page. Just navigate to any project and scroll down the the section titled "Your Notes".
Similar to the client notes, these are only visible to you and your team, not to the client.