Use in-product help to clarify a UI, such as what a feature does, or what a specific term means. It should be written for novice users, and easy for expert users to ignore.
Use help text to provide a one or two line description of what a feature does, with a link to more information in the Knowledge Base. The link text should clearly explain where it will take users. It will typically take the form, "Learn more about [x] in the Knowledge Base."
Use help bubbles to provide unobtrusive clarifying information. Help bubbles are great for saving space and de-cluttering a layout, but only when the information isn't required to complete a task. For example, experts don't need an explanation of what "Segmentation" is every time they build an audience.