Coma aberration which means "comet" in latin, is similar to spherical aberration; it applies to rays entering the lens at an angle. You probably created this aberration being kid by tilting a lens under the sunlight. At the beginning the projected image of the sun is circular but as you tilt the lens with respect to the sun direction the resulting image takes an elongated shape, like a comet.
This coma aberration is dependent upon lens shape. Rays incoming from the periphery of the lens focus closer to the axis and produce a larger blurry spot than the paraxial rays. As coma is proportional to the distance to the central axis, more the rays are away from the center, more the focal point changes of position and get blurry images, mainly off-axis.
For a newtonian the length of the coma is expressed as L = (3/16)(D/F)2a, where D is the primary mirror diameter, F the focal length and a the distance to the axis. For the Palomar telescope of 5.08 m used in newtonian configuration, the focal length is 16.3m. The relation tell us that L = a/55. That means that at 55" for the axis the coma is already 1" long ! To stay below that resolution, the useful field is only 9mm wide...
This effect is the most obvious on fast scopes like dobsonians or poor quality astrographs. SCT will all their spherical mirrors display also an important coma larger than an optimized design with one or two aspherical or aplanetic mirrors (Ritchey-Chrétien, etc). Commercial designs could perform significantly better scopes but this is not often the case for financial reasons.
Coma can be corrected by using corrective lenses placed symmetrically around the axis. Ross or Wynne corrector using two to four lenses are built to correct this aberration as well as the others. For visual observations, this effect can be suppressed using for example the Tele Vue Paracorr suited to newtonian scopes.