In the United States, winking is a guy's way of letting a girl know he finds her attractive. It is a form of flirting. Among friends, it is also a way for a guy to communicate that he is teasing.
When interpreting a wink, it is important to put it into the context of the situation. A guy who winks at a girl he barely knows is likely trying to communicate that he is interested and would like to get to know her better. It is a way of initiating contact and inviting the girl to approach him. A guy who winks at a girl with whom he is in a romantic relationship is usually trying to tell her that he thinks something she has done or said is particularly cute or attractive. When a guy winks at a girl with whom he is friends and in whom he has no romantic interest, it is a way of letting her in on a joke or a tease. It is important to remember, however, that these are only meant as interpretations of winking in the United States. In other countries, particularly Asian nations, winking is an offensive gesture and is discouraged.