SEC filings are company reports filed with the Securities & Exchange Commission. The agency began requiring electronic submission of reports in the 1990s. With some exceptions, all companies were filing electronically by early 1996. Some companies' reports are available back to 1993.
EDGAR, Electronic Data-Gathering, Analysis, and Retrieval, is the name of the system/database of reports, http://www.sec.gov/edgar.shtml.
Filings from the SEC may be obtained directly from EDGAR on their website, http://www.sec.gov. Search by company name (first option). After you've selected your company from the list of results, you'll have the option to filter the results by filing type.
Some of the more useful forms:
For a complete list of forms, go to http://www.sec.gov/info/edgar/forms/edgform.pdf.