Look for the string of letters or numbers following the letters "TV." Those are the ratings. Every rating starts out with "TV," then follows with a series of letters of numbers that briefly describes the content of the program.
Check the first set of letters or numbers listed after "TV" in the rating. They contain the essence of the rating, and there are six different types. "Y" means a show aimed primarily at younger children, like "Sesame Street." "Y7" is still aimed at children, but mainly older children in grade school; "Batman" or "Spongebob Squarepants are good examples. "G" means a show not specifically aimed at children, but that is suitable for them nonetheless: a nature documentary, for example. "PG" is probably still okay for children, but may contain more grown-up content (a lot of sit-coms match this rating), while "14" means that parents with children under 14 should seriously consider whether the show is appropriate (network courtroom dramas, for example). "MA" means "mature audiences" and contains material equivalent to an "R" or an "NC-17" rating at the movies. It includes grown-up cable shows like "The Sopranos" and parents should probably wait until the kids go to bed before watching it.
Look at the last letters in the rating. They provide more detailed information about the content, which allow parents to make more informed choices than the ratings alone. For instance, a given parent may have no worries about bad language, but doesn't want his child watching sex. Or he may not concern himself with sex, but doesn't want his kids watching violence. The last series of numbers covers those circumstances. Specifically, the "TV-Y7" rating may sometimes be followed by an "FV," which stands for "fantasy violence." That means the show contains a certain amount of bloodless violence, tantamount to Batman punching a bad guy in the jaw. The other letters will appear only for shows rated "TV-PG," "TV-14," or "TV-MA." "D" refers to suggestive dialogue--not flat-out bad language, but likely containing references to sex. "L" means bad language, "V" means violence, and "S" means sex.