Originally cat charms were used to signify luck and are common today. The question is whether they represent good or bad luck.
The black cat, in particular, is sometimes seen as unlucky as it represents one of the forms Satan is believed to take as he stalks the Earth. Others will wear the black cat charm to specifically ward against bad luck.
The Romans took the fiercely independent nature of the cat and translated this to the concept of liberty. The ancient Egyptians associated the cat to the moon and drew connections between the dilation and contraction of cat eyes to the waxing and waning of the moon. Egyptian moon gods or goddesses would often be depicted with the wedjat eye in the full-moon disk over their head. The wedjat-eye, however, belonged to Re and was later associated with Thoth, rather than the cats.