|This woman has heterochromia,with one brown eye and one hazel eye.|
There are multiple ways in which heterochromia can be classified (and the classification I give here may be revised in the near future). There is complete heterochromia, where each eye is fully of one color. So, for example, one eye is blue and the other is brown or one eye is amber and the other green, among numerous other possibilities. This can be either inherited or acquired, with the acquired form having numerous different causes.
|Central heterochromia in my own eye|
Causes are incredibly varied when it comes to heterochromia. When the condition is present from birth (congenital), it is overwhelmingly likely that it is an inherited trait in some form or another. Any of the types of heterochromia can be congenital. Sometimes the inheritance is obvious (such as another family member having the same trait), but other times it is not. Familial heterochromia (there the trait is inherited and runs in a family) is a rather common explanation. Others are partial albinism (complete absence of pigment in a small area of the body), piebalding (a form of dominant lack of pigment sometimes seen in humans), and the potential for congenital heterochromia to be caused by mosaicism (mutation) or chimerism (fusion of twins early in development).
|I suspect Jen's dark spots are from injury|
|A cat with complete heterochromia|
EDIT: I forgot to add to this in. There are numerous celebrities nowadays who have heterochromia. Some are more famous than others, but all have fascinating eyes. Mila Kunis, Christopher Walken, Kate Bosworth, Dan Akyroyd, Max Scherzer and Jane Seymour all have heterochromia of various types, just to name a few. David Bowie is thought by some to have heterochromia, but in fact has a permanently dilated pupil.
Sources are Right Diagnosis, Sewanee University of the South, PubMed Central from the National Center for Biotechnology Information, Miles Research, h2g2, University of Maryland Medical Center, Indian Journal of Ophthalmology, and Wikipedia. Images except for my eye and Jen's eye are from Wikimedia Commons under Creative Commons licenses: one, two, three (mine), four, five (mine), six.