What is the difference between an A-mount and an E-mount?

For interchangeable lens cameras produced by Sony, there are two types of mount standard: A-mount and E-mount.

The mount is the part that joins the camera’s body and the lens. To shoot images, attach the lens to your camera’s lens mount and let the lens and the body recognize each other.

• A-mount (ILCA-xx, DSLR-Axx, SLT-Axx)
  Mount standard adopted on models that have a mirror or a translucent mirror.
• E-mount (ILCE-xx, NEX-xx)
  Mount standard adopted on mirrorless models.

[1]Mount on an A-mount body
[2]Mount on an E-mount body

• A-mount and E-mount features
  A-mount cameras with Sony Translucent Mirror Technology offer the advantages of full-time phase-detection autofocus. The short flange back distance of mirrorless E-mount models makes it possible to design compact, high-performance bodies and lenses that offer outstanding handling and mobility.

  A-mount [1] Lens mount  [2] Flange back distance [3] Translucent Mirror  [4] Image sensor plane

  E-mount [1] Lens mount  [2] Flange back distance [3] Image sensor plane

• Image sensor size
  Both A and E mount cameras have full frame and APS-C sensors options. Among the many sensor sizes in common use, full-frame is considered to be a large format while APS-C is a medium format. Both are popular for their individual advantages.
  
• About A-mount lenses and E-mount lenses
  Sony provides A-mount lenses and E-mount lenses that meet each mount standard for camera bodies. The name of an A-mount lens starts with “SAL” while the name of an E-mount lens starts with “SEL.”
  
  For lenses or accessories compatible with Sony cameras, refer to FAQ:
  Lens and accessory compatibility for cameras and camcorders
  
  For lenses or accessories compatible with other brands other than Sony, refer to FAQ:
  Can I use lenses and accessories manufactured by companies other than Sony?
•  
• How to use an A-mount lens on an E-mount body
  Since A-mount and E-mount have different diameters, they are not interchangeable. However, you can use an A-mount lens on an E-mount body by using a Mount Adaptor.
  NOTE: You cannot use an E-mount lens on an A-mount body.
  
  When you use a Mount Adaptor, there may be some limitations such as unavailability of the autofocus function or SteadyShot function.
  
  For the differences among Mount Adaptors, refer to FAQ
  What is the difference between the LA-EA1, LA-EA2, LA-EA3, and LA-EA4 lens mount adapters?