Female anime or manga characters with cat ears and sometimes tail. One of the best-developed (in terms of personality, not anatomical features, you dirty otaku) is probably Artemis Alpeia from the manga of "Appleseed", by Masamune Shirow.

An otherwise normal human-being female with cat-like features, such as those cute ears and a tail. Catgirls also tend to have other psychological features of cats; I'm sure you can use your imagination here ;)

See also neko, miao.