In 1996, Congress required video program distributors (cable operators, broadcasters, satellite distributors and other multi-channel video programming distributors) to close caption their television programs. Pursuant to this requirement, the FCC in 1997 set a transition schedule requiring distributors to provide an increasing amount of captioned programming.

The FCC mandated that most Broadcasts and some Commercials have Closed Captioning by
January 1, 2006.

Styles of Closed Captioning
There are three styles of line-21 caption display currently in common use. Pop-On, Roll-Up and
Paint-On. All captions can be strategically placed (left, right, center, top, bottom) on the screen for the viewer to understand who is saying what and when. Below you will see the difference between the three styles.

Pop-On Captioning
Pop-On captioning is the most commonly used captioning style for Non-Live Broadcasts. A Pop-On caption commonly contains between one and four rows of text. The text is shown all at one time and is synced with the audio. Once the audio and captions are complete, new lines of captions will appear on the screen.
Example:

Roll-Up Captioning
Roll-Up captioning is almost exclusively used for Live Broadcasts. The captions are verbatim. Each new sentence begins a new row, and each speaker change is indicated with a speaker-change symbol (>>). A new caption row is generally displayed just as the speaker begins to say the first words in the row. Each word is shown as it is spoken. Once the rows are filled with text, the rows will "Roll-up" by one line and a new text line will start.
Example:

Paint-On Captioning
Paint-On captioning is rarely used today however, it's still available. Take the same principles of Roll-Up captioning and instead of the captions "Rolling Up" after the text lines are filled, they will disappear and start showing the next text line one word at a time.
Example: