Captions provide equivalent text for individuals who are unable to hear the audio portion of a video presentation. Open captions are part of the video and can not be turned off. Closed captions, however, are contained with a separate text track and can typically be toggle on or off as needed.
Audio description is a technique for making video accessible to individuals who are blind. If important information in a video is presented visually but is not obvious through the program's audio, this information must be described by a narrator and made available in a separate track for individuals who need it.
Media players vary in their support for closed captions and audio description, as well as in the steps users must follow to turn these features on when they are available.
Many media players today are embedded into web pages, and are most commonly developed for Adobe Flash or Microsoft Silverlight browser plug-ins. Tools are readily available for developing in these environments, and as a result many developers have created custom video players or "skins". Unfortunately many of these players do not include support for closed captions, but a few do. Two of the most popular Flash video players that support closed captions are JW FLV Player  from LongTail Video and ccPlayer  from the National Center on Accessible Media. Each of these players includes a "CC" button on the controller bar for toggling captions on and off. In early versions of the JW FLV Player (prior to version 4.0) the closed caption button was labeled with a "T" for "text". In all versions of the JW FLV Player, this button is only shown if captions are available, and only if the web developer chooses to include it.
JW FLV Player provides similar support for audio description. If audio description is available and implemented, an audio description button is provided on the controller bar, labeled with the letter "A". This button allows users to toggle the narration on and off. The narration track itself is provided in an MP3 audio file, which is synchronized with the video program by the player controller.
YouTube supports closed captions, but not audio description. If captions are available, they can be turned on by clicking the button in the lower right corner of the YouTube video player. This activates a pop-up menu from which "turn on captions" can be selected. Once captions have been turned on in the YouTube player, they remain on indefinitely unless the user follows this same procedure to turn them off. The YouTube player also provides user with control over the display of captions, including font size and background. These options are explained on YouTube's Captions and Subtitles  page.
In standalone media player software applications such as Apple iTunes, Microsoft Windows Media Player, QuickTime Player, and Real Player, captions and (if supported) audio description are typically activated by selecting relevant options in the application menu. The precise wording and location of these options varies across products and versions. Below are a few examples:
Windows Media Player version 9 and higher (Windows)
Windows Media Player does not currently support descriptive video. It does support closed captions. However, there are several requirements for closed captions to be displayed:
- Windows Media Player must be in Full Mode (Control+F1), rather than Skins Mode. Windows Media Player has a variety of visual skins available, but few support the display of closed captions.
- Turn on captions by performing the following steps:
- Press Alt+P to activate the Play menu (this hotkey combination will work even if the application menu is not visible)
- Select Captions and Subtitles
- Select On if Available
- In version 10 and higher, an additional step required in some circumstances because Windows Media Player's security settings block the display of captions by default. To assure that the display of captions is permitted, perform the following steps:
- Press Alt+T to activate the Tools menu (this hotkey combination will work even if the application menu is not visible)
- Select Options
- Select the Security tab
- Check the box beside "Show local captions when present"
- Select OK
QuickTime Player 7.2 and higher (Windows or Mac)
In QuickTime Player, all captions are open captions (if available, captions are always visible on the screen). The same is true for audio description.
- In the Windows version, select Edit > Preferences > Player Preferences. On the General tab, select "Show closed captions when available".
- In the Mac version, select Quicktime Player > Preferences. On the General tab, select "Show closed captions when available".
Real Player version 9 and higher (Windows and Mac)
- In the Windows version, select Tools > Preferences from the menu bar.
- In the Mac version, select Real One Player™ > Preferences from the menu bar.
- Several Preferences categories appear in the left column. From these categories, select Content.
- Under Content, there is a section titled Accessibility, which includes two checkboxes, one for toggling captions, and one for toggling descriptive audio.
Apple iTunes version 9
- Select Edit > Preferences. This opens a Preferences box. From this box, select the Playback tab, and check the box labeled "Show closed captions when available".
- Also, select Control > Audio & Subtitles, then select the desired language or option. This option is grayed out unless the video you are currently viewing has captions available.
To turn on closed captions on an iPhone or iPod touch, go to Settings > iPod and move the Closed Captioning slider to "on."
To turn on closed captions on an iPod nano, go to Videos > Settings and select "Captions".
Since so few of the players described in this Knowledge Base article provide support for closed audio description, the best solution currently is to premix the narration track into the video, and provide two versions of the video, one with audio description and one without.
To learn how to provide captions on videos, see the Knowledge Base article What types of closed caption files do video players support? . For more a more general overview of multimedia accessibility, see the Knowledge Base article How do I make multimedia accessible? 
-  JW FLV Player
-  ccPlayer
-  YouTube's Captions and Subtitles
-  What types of closed caption files do video players support?
-  How do I make multimedia accessible?