What do you need to know to best adapt your application letter to a specific organization?

You'll need to know the following:
The name and address of the person who should receive the letter. To get this information, check the advertisement, call the organization, check its website, or check with your job search contacts. An advantage of calling is that you can get current information and find out what courtesy title the individual prefers.
What the organization does and some facts about it. Knowing the organization's larger goals enables you to show how your specific work will help the company meet its goals. Useful facts can include market share, new products or promotions, the kind of computer or manufacturing equipment it uses, plans for growth or downsizing, competitive position, challenges the organization faces, and the corporate culture.
What the job itself involves. Campus placement offices and web listings often have fuller job descriptions than appear in advertisements. Talk to friends who have graduated recently to learn what their jobs involve. Conduct information interviews to learn more about opportunities that interest you.