Playful banter usually, but not restricted, with members of the opposite sex. This usually includes a conscious desire to flatter the one being flirted with. Flirting is a core aspect of many relationships today between a boy and a girl.
Girls are often caught flirting with each other. In this context, it is usually not intended sexually but playfully.
Nearly all accounts of flirting include subtle sexual innuendos and, to the unexperienced, can sometimes seem harmless.
BAD: "LAWL YOU'RE HAWT"
GOOD: "Guuuurl, if you were Sprite, I'd obey my thirst ;)"
Winking is an effective means of alerting participating parties that you are flirting