A Z-Lister is one of the least well-known celebrities; a celebrity well past their peak of appeal. They are a notional list of the least celebrated or sought-after members of a group, especially in the entertainment industry. As they appeared in either movies, TV series or done very little in general.
For example: Jon Malkovich is a Z-List celebrity just because he was in one of the infamous Star Wars movies, but only as an extra. This shows that people would recognise him for his "work", not for who he really is, nor his actual involvement in the feature.
A-List celebrities are people who have vast wealth, connections, and influence within there respective industries would be considered an A-list IMO, examples being Jay Z, Lady Gaga, Eminem, Simon Cowell, Ant and Dec, Beyonce, Oprah Winfrey, James Cameron, Robert Downy, Will Smith, Bill Gates, J K Rowling, The Queen, Obama etc
A Z-list celebrity would be someone who has built a career (or lack there off) on what would be considered a lack of discernible talent or notoriety. Examples being from '#2828228 reality show', Katie Price, Paris Hilton, Kerry Katona, Peter Andre, Tila Tequila, Frankie Cocozza, Jodie Marsh, Eric The Midget etc