The wink is an intentional facial expression made by briefly closing one or both eyes (http://dictionary.reference.com/browse/wink). To wink is to close and open either one or both eyelids with a rapid motion; to blink suggests a sleepy, dazed, or dazzled condition in which it is difficult to focus the eyes or see clearly. A wink is a form of semi-formal communication, which indicates shared, unspoken knowledge.
A "naughty wink" can silently indicate a shared secret, such as if a salesperson gives a customer a brochure and says, "Here you go; it's free". Infrequently, it may also mean "got it" or "yes, I understand".
In Western cultures, women may wink to men they are interested in dating, but this has grown out of fashion, though still used occasionally. Winking is also done by men to women, often to convey a message of "I like what I see here" or "Hello, I am interested in getting to know one another if that is agreeable with you."
In Latin American cultures, winking is also a romantic or sexual invitation.  In Nigeria, winking is a signal for children to leave the room.  Many Asians, especially Chinese and Indian women, consider winking to be rude. .
Not all humans are able to wink voluntarily, and some can only wink one (usually the non-dominant) eye but not the other whilst others are far better at winking one eye and find it awkward to wink the other.