What is a rating and a share?
The terms rating and share are basic to the television industry. Both are expressed as percentages.

Simply put, a rating tells how many people watched a particular TV program; it is the percent of households or persons within a universe (all TV households, or adults 18-49, for example) who are tuned to a particular program or daypart.

A share expresses this same number of viewers as a percent of only the households or persons actually watching television during the program or daypart. Thus, a share is a percent of a constantly changing number — the number of homes or number of persons in a given demographic using television at that time. Shares can be useful as a gauge of competitive standing.

Source : Nielsen Media Research