The middle finger is a big, bold sign of dissatisfaction and anger, a gesture that pokes both the giver and the recipient's inner beast that might just come out in retaliation, resulting in all hell breaking loose.
Flashing your middle fingers in alternating fashion (like what the lady above does) actually means "siblings" in Japanese sign language. While sticking up one finger can be translated as "ani" which means "older brother."
The other fingers also represent different meanings. According to Kotaku, the thumb means "father," the index means "mother," the ring finger means "sister," and the pinky means "baby."
FYI: The middle finger also has a similar meaning in China and Korea, as you can see below:
We have to warn you though: We recommend not doing this:
GIF via Gifsoup.com
...in the streets of Japan, just to be safe. They may represent something harmless in the Land of the Rising Sun, but we're pretty sure the Japanese are aware of its other, less-innocent meaning. Two words: Hollywood movies.