Japan introduced school uniforms in the late 19th century. Today, school uniforms are almost universal in the public and private school systems. They are also used in some women's colleges. The Japanese word for uniform is Seifuku.
The sailor outfit is a common style of uniform worn by female middle school and high school students, and occasionally, elementary school students. It was introduced as a school uniform in 1920 in Heian Jogakuin and 1921 by the principal of Fukuoka Jo Gakuin University ,Elizabeth Lee. It was modeled after the uniform used by the British Royal Navy at the time, which Lee had experienced as an exchange student in the United Kingdom.
Much like the male uniform, the gakuran, the sailor outfit bears a similarity to various military styled naval uniforms. The uniform generally consists of a blouse attached with a sailor-style collar and a pleated skirt. There are seasonal variations for summer and winter: sleeve length and fabric are adjusted accordingly. A ribbon is tied in the front and laced through a loop attached to the blouse. Several variations on the ribbon include neckties, bolo ties, neckerchiefs, and bows. Common colors are navy blue, white, grey, light green and black.
Shoes, socks, and other accessories are sometimes included as part of the uniform. These socks are typically navy or white. The shoes are typically brown or black penny loafers. Although not part of the prescribed uniform, alternate forms of legwear (such as loose socks, knee-length stockings, or similar) are also commonly matched by more fashionable girls with their sailor outfits.
Various schools are known for their particular uniforms. Uniforms can have a nostalgic characteristic for former students, and is often associated with relatively carefree youth. Uniforms are sometimes modified by students as a means of exhibiting individualism, including lengthening or shortening the skirt, removing the ribbon, hiding patches or badges under the collar, etc. In past decades, brightly coloured variants of the sailor outfit were also adopted by Japanese yankee and Bōsōzoku biker gangs.
Because school uniforms are a popular fetish item, second-hand sailor outfits and other items of school wear are brokered through underground establishments known as burusera, although changes to Japanese law have made such practices difficult. The pop group Onyanko Club had a provocative song called "Don't Strip Off the Sailor Suit!" Sailor outfits, along with other styles of school uniform, play an undeniably large role in otaku culture and the Japanese sexual canon as evidenced by the large amount of anime, manga, and dōjinshi featuring characters in uniform, Sailor Moon being one of the most popular examples.
Japanese schoolgirls, as the label, not simply girls attending a japanese school, are distinguished by the fact that they wear their uniforms outside of school, becoming a fashion in it's own right, much like the 'Cafe Maid' craze which followed. As a rule, they listen to whatever music is popular in japan at that time, so jPop, currently. They probably also like some shojo anime. It is not just the uniform, of course, as no-one has only one outfit, and outside of their seifukus japanese schoolgirls dress girly-ish, with short skirts, frilly babydoll tops, cute plushie backpacks, and lots of ribbons. They likely dream of being idol singers, and are giggly, bubbly, and often a little scatterbrained.
Info from Wikipedia, last paragraph by me. Image made on a dress up game on Deviantart.