Custom block types are like mini content types, as we can add fields to them for specific purposes.
These are the ones that currently exist:
Alert
Image
Video
Alert
Alert blocks provide an area to add text and a selector to choose the “background“ color. These use Bootstrap alerts styling.
Image
Image blocks provide the ability to upload an image, add an optional caption, and a provide link (that will wrap the image on display).
Video
Video blocks provide a video field to embed YouTube or Vimeo videos.