It aims to emulate the ed editor, but with extended capabilities, such as editing multiple files at once. It does this by opening a window dedicated to "watching" each file, and standard ed commands can be, in a sense, directed to the current file set.
Real editing power is offered by Plan 9's mouse chording capabilities, however this is available to any Plan 9 window, whether terminal or editor, not just sam.
wikipedia.org dumped 2003-03-17 with terodump