Visas and Programs for Foreign Students in Japan. Study in Japan (Government of Japan)
A website, Japan-Guide, offers a one-page summary of the different types of visas and categories.
Here is a link to an organization that arranges and promotes internships for companies in Japan, but this website is almost all in Japanese.
Finally, here is a link to the Consular information section on Visas from the Government of Japan
Here is a summary from a company that provides visa processing and assistance services on visas for Entrepreneurs.