Flirting...

I don't have a lot to say on this, wikipedia does a good job at it I think (as does the others)...

Flirting is a form of human interaction, usually expressing a sexual or romantic interest in the other person. It can consist of conversation, body language, or brief physical contact. It may be one-sided or reciprocated.

Flirting is often used as a means of expressing interest and gauging the other person's interest in courtship, which can continue into long term relationships. Alternatively, it may simply be a prelude to casual sex with no continuing relationship. A person who takes flirting to the opposite extreme is often considered promiscuous or a slut.

In other situations, it may be done simply for immediate entertainment, with no intention of developing any further relationship. This type of flirting sometimes faces disapproval from others, either because it can be misinterpreted as more serious, or it may be viewed as "cheating" if the person is already in a romantic relationship with someone else.

People who flirt may speak and act in a way that suggests greater intimacy than is generally considered appropriate to the relationship (or to the amount of time the two people have known each other), without actually saying or doing anything that breaches any serious social norms. One way they accomplish this is to communicate a sense of playfulness or irony. Double entendres, with one meaning more formally appropriate and another more suggestive, may be used.

Phychology Today tells this on a study of flirting:

This much is clear so far: flirting is nature's solution to the problem every creature faces in a world full of potential mates-how to choose the right one. We all need a partner who is not merely fertile but genetically different as well as healthy enough to promise viable offspring, provide some kind of help in the hard job of parenting and offer some social compatibility.

Our animal and human ancestors needed a means of quickly and safely judging the value of potential mates without "going all the way" and risking pregnancy with every possible candidate they encountered. Flirting achieved that end, offering a relatively risk-free set of signals with which to sample the field, try out sexual wares and exchange vital information about candidates' general health and reproductive fitness.

"Flirting is a negotiation process that takes place after there has been some initial attraction," observes Steven W. Gangestad, Ph.D., an evolutionary psychologist at the University of New Mexico in Albuquerque who is currently studying how people choose their mates. "Two people have to share with each other the information that they are attracted, and then test each other" on an array of attributes. Simply announcing, 'I'm attracted to you, are you attracted to me?' doesn't work so well. "It works much better to reveal this and have it revealed to you in smaller doses," explains Gangestad. "The flirting then becomes something that enhances the attraction."

It is an axiom of science that traits and behaviors crucial to survival-such as anything to do with attraction and sex-require, and get, a lot of an animal's resources. All mammals and most animals (including birds, fish, even fruit flies) engage in complicated and energy-intensive plots and plans for attracting others to the business of sex. That is, they flirt.

Social issues research Centre says this:

Clearly, it makes sense to exercise a degree of caution with people who are married or attached... couples differ in their tolerance of flirtatious behaviour, and it is important to be alert to signs of discomfort or distress.

Ann Landers said this about flirting in an article in the Post:

Does it not subliminally indicate that you are interested in other people? I don't understand where people try to act like it doesn't mean anything. If your in a serious relationship and your partner is constantly seeking the attention of the opposite sex then how does that show people he's content and happy with his current relationship? People could interpret that the relationship is vulnerable. Part of being in a relationship is letting your partner feel like you are committed and letting everyone else know it too. Isn't that part of honoring and respecting the commitment?