In 1997, television shows were coded as:
• TV–G — Suitable for all ages; little or no violence, strong language. or sexual situations.
• TV–PG — Parental guidance suggested; may be unsuitable for younger children.
• TV–14 — Parents strongly cautioned; may be unsuitable for children under 14.
• TV–MA — Mature audiences only; may be unsuitable for children under 17.

As a result of organizations like the National PTA and other child advocacy groups, television networks now offer content-based codes that give parents more information regarding the material of television programs. The following content-based codes are in addition to the previously mentioned codes:
S — sexual situations
V — violence
L — coarse language
D — suggestive dialogue
TV–Y — children of all ages
TV–Y7 — children older than 7
FV — “fantasy violence” in programs for children 7 and older.

Television shows are rated by the networks and producers. A monitoring board consisting of experts of the TV industry and child advocacy groups examines the programs to ensure they have been correctly coded.

References:
http://fun.familyeducation.com/television/parenting/35047.html
www.wbgn.com/tv_rating_info.htm