Taken directly from the version of Adobe Acrobat Reader Help that I have installed, and since you didn't indicate the version used on your system I hope the following makes sense -- except the paragraphs are not numbered in the help section itself:
1. Setting Options preferences: The Options panel of the Preferences dialog box provides the following preference options:
Display PDF in Browser (Windows only): Displays any PDF document opened from the Web inside the browser window. If this option is not selected, PDF documents will open in a separate Acrobat Reader window.
Auto-configure Browser (Mac OS only): Ensures that your Web browser is configured to work with Acrobat Reader each time Acrobat Reader is launched.
Check Browser Settings When Starting Acrobat: Checks your default browser settings for compatibility with Acrobat Reader each time Acrobat Reader is launched.
Allow Fast Web View: Downloads PDF documents for viewing on the Web one page at a time. If this option is not selected, the entire PDF will download before it is displayed. If you want the entire PDF document to continue downloading in the background while you view the first page of requested information, be sure Allow Background Downloading is also selected.
Allow Background Downloading: Allows a PDF document to continue downloading from the Web, even after the first requested page displays on-screen in a browser.
Display Splash Screen: Shows the splash screen each time Acrobat Reader is launched.
Certified Plug-ins Only: Loads only Adobe-certified third-party plug-ins. If you use non-certified plug-ins for Acrobat Reader, make sure that you select this option to use the Web Buy feature or to open documents with additional usage rights.
Use Page Cache: Places the next page in a buffer even before you view the page in Acrobat Reader. This reduces the amount of time it takes to page through a document.
2. Viewing PDF documents on the Web:
Acrobat Reader 5.1 installs plug-ins that make viewing PDF documents on the Web easy. You can view PDF documents in your browser, or set up Acrobat Reader to work as a helper application, so that when you open or download PDF documents from the Web, they open in a separate Acrobat Reader window.
3. Reading PDF documents in a Web browser:
When a PDF document is stored on the Web, you can click a link to open the document in your Web browser. PDF documents can display in Web browsers compatible with most versions of Netscape Navigator