Literally translated, manga means "random pictures." The word first came into common usage after the publication of "Hokusai manga," containing assorted drawings from the sketchbook of famous ukiyo-e artist Hokusai. Though roughly equivalent to the American comic book, manga hold more importance in Japanese culture than comics do in American culture. Manga is much respected both as an art form and form of popular literature.
Manga magazines usually have many series running concurrently with approximately 30-40 pages allocated to each series per issue. These manga magazines, or "anthology magazines," as they are also known, can be anywhere from 200 to more than 850 pages long. Manga magazines also contain one-shot comics and various four-panel manga (equivalent to comic strips). Manga series can run for many years if they are successful.
When a series has been running for a while, the stories are collected together and printed in dedicated book-sized volumes. These volumes use higher-quality paper, and are useful to those who want to "catch up" with a series so they can follow it in the magazines or if they find the cost of the weeklies or monthlies to be prohibitive.
Manga have been translated into many different languages in different countries including China, France, Italy, and many more. In the USA, manga is still a rather small industry, especially when compared to the inroads that Japanese animation has made in the USA. The leading manga publisher in America is Viz, the American branch of publisher Shogakukan http://www.shogakukan.co.jp/ (小学館). They have many popular titles such as Neon Genesis Evangelion, Dragon Ball Z, Tenchi Muyo and the various works of Rumiko Takahashi.
The most popular and recognizable style of manga is very distinctive. Emphasis is often placed on line over form, and the storytelling and panel placement differ from those in western comics. Panels and pages are typically read from right to left, consistent with traditional Japanese writing. While the art can be incredibly realistic or cartoonish, it is often noted that the characters look "Western", or have large eyes. Large eyes have become a permanent fixation in manga and anime since the 1960s when Osamu Tezuka, creator of Astro Boy and considered the father of modern manga, started drawing them that way, mimicking the style of Disney cartoons from America. Being a very diverse artform, however, not all manga artists adhere to the conventions most popularized in the west through anime such as Akira, Sailor Moon, Dragonball Z and Ranma 1/2.
Fairly surprising for western readers is that (somewhat like the Jazz approach to melody) manga artists don't feel that their stories and characters are set in stone. So a set of characters may build relationships, jobs, etc. in one set of stories ("story arc") only to have another story arc run where the same characters do not know each other. The "Tenchi" series in particular is known for this; there are more than thirteen different pretty-much unrelated story arcs based around Tenchi and his friends.
Manga, sometimes even adult manga, often have furigana. The purpose of furigana in manga is to help younger children who are still learning how to read complex kanji. They can read the simple-character furigana earlier because it is taught in school earlier than the complex characters are. There are even special furigana Japanese-English dictionaries. Furigana is also often used for character names, where unusual kanji are used or the name may have more than one possible reading.
Taking a look at online scanlations of manga is a good way to experience the genre.
Some manga artists will produce extra, sometimes unrelated material, which are known as omake (lit. "bonus" or "extra"). They might also publish their unfinished drawings or sketches, known as oekaki (lit. "sketches").
Unofficial fan made comics which continue with a series' story or write an entirely new one using its characters also exist. They are known as doujinshi.