In Korea they call their comics manhwa and in China they're called manhua. Manga artists are called Mangaka.
In an extremely broad sense the common manga will contain either schoolgirls, robots, cutesy animals or monsters. Many westerners think that all mangas contain pornographic material, which is obviously untrue.
This can be insulting to many manga artists.
In 1945ish splits appeared in the styles of manga. This is to do with the US invasion of Japan. Some artists took inspiration from the comic books the Americans brought over with them and cartoons introduced, such as Disney cartoons.
Other artists, notably Kinko Ito, stayed true to the Japanese cultures of comic writing and drawing. However, many hybrid versions were also created. The modern mangas popular in America, the UK and France would be a mixture of the two styles.
One of the most revered figures in manga and anime is Osamu Tezuka. He's referred to as the 'Godfather of Anime' and the 'God of Manga'.
He created Astro Boy which has received numerous awards and is internationally renowned.
Mangas are nearly always in black and white and is published in binded volumes, unlike typical western comics, which tend to be colourful and sold in thin booklets. Despite their durability, they aren't that much more expensive, making collecting them much more practical than collecting comics.
A few more things.
pictures:
http://upload.wikimedia.org/wikipedia/en/5/54/AstroBoyVolume1.jpg
http://graphjam.files.wordpress.com/2009/08/song-chart-memes-japanese-population.jpg