Imagine Stan Lee, Jack Kirby, and Walt Disney rolled up into one person and that about estimates how important Osamu Tezuka is and was to Japanese comics. The man invented and popularized many of the conventions of manga and went on to produce a prodigious number of anime as well at the Mushi Production and Tezuka Productions studios.
"The God of Comics" was a a trained medical doctor who forwent practicing in order to follow his passion and make comics. In post-World War II Japan much of the industry had to be rebuilt and Tezuka's tireless efforts were a beacon in the manga and anime worlds. He worked on 700 manga series in his lifetime and produced about 150,000 volumes of work, most of which are only available in Japanese. Among his most famous series are Astro Boy, Phoenix, Blackjack, Buddha, and Kimba the White Lion, which some people claim is the direct inspiration of Disney's The Lion King. To be fair, Disney was a major influence on his work as well. Additionally, many of his works were also turned into anime series and he worked on at least 60 films. Much of his work is covered in more detail on the Tezuka in English fansite.
Tezuka's life and work are commemorated by two awards, the Osamu Tezuka Awards and the Osamu Tezuka Culture Award, both given for achievements in manga. If you are ever in Japan, you can also visit the Osamu Tezuka Manga Museum, located in Takarazuka, the city where he was raised.