EDGAR is the SEC's Electronic Data Gathering and Retrieval system. You can find most financial documents filed with the SEC on this website. For details about what EDGAR does and does not include, please see Important Information About EDGAR.
There are numerous forms that businesses must regularly file with the SEC. You will commonly see refereces to the 10K, which is the annual report of a company reuqired to be filed by the end of the company's fiscal year. Other commonly referred to forms are the 10Q (quarterly report) and the 8K (update to the 10K and 10Q forms).
It is very easy to search for documents within EDGAR. For a brief tutorial, please see the SEC's EDGAR Quick Tutorial.
The easiest way to get started is to look up information by company name or ticker symbol using the Company Search page. In the example below, I am searching for documents filed by Apple using the ticker symbol AAPL.
Your results page will show filed documents in reverse chronological order (i.e. most recently filed appear first). Click on the "Documents" or "Interactive Data" links to view the information.
Once you are on the results page, you can filter the results. In the example below, I limited the documents to 10-K forms only.