While the term "annual report" is often used to describe either a Form 10-K or an Annual Report to Shareholders (or ARS), there is a difference.
A public company's 10-K (or 20-F for certain foreign companies) is an excellent source of information on its business, strategies, risks, financials, and more. It presents the company's point of view on its current condition and future direction. Other SEC filings reveal important information as well.
Recent and some historical annual reports are usually available on a company's website. To find them, simply google [company name] investor to get to the Investor Relations (or sometimes just Investors) section of the site. There should be a section on annual reports or SEC filings.
10-Ks are available online on the SEC site (see "Recent Annual Reports" on the left) back to 1994. 10-Ks (or information from the 10-Ks) before that may be available from the following sources: