The actors whose names appear first are said to have "top billing". They usually play the principal characters in the show/film and have the most screen time.
In film it is possible to give actors "equal billing" by placing their names side-by-side however this is not practical on a much smaller TV screen, thus this leads to...
"last billing", which is usually credited after the rest of the lead cast, prefixed with "and". In some cases, the name was followed by "as" plus the name of the character.