Hubble's Tuning Fork diagramDraw Hubble's Tuning Fork diagram
Edwin Hubble classified galaxies according to their shape. He produced a tuning fork diagram as he thought this demonstrated the evolution of elliptical galaxies into spirals.
This theory is considered to be incorrect but astronomers still use the diagram and his classification to judge what category a galaxy fits under.
Elliptical galaxies are classified from E0 (for spherical galaxies) to E7 (for very elongated).
Spirals are classified Sa, Sb, Sc. A means that the spirals extend from the nucleus outwards with a slight bend. C means the spirals are wrapped around the nucleus tighter.
Spiral Barred, or Barred Spiral depending what book you read, are classified likewise SBa, SBb and SBc.
Irregular galaxies are not part of the Hubble Classification.