Answer by taking a quote from the following article:

RuPaul's Drag Race is an American reality competition television series produced by World of Wonder for Logo TV and, beginning with the ninth season, VH1. The show documents RuPaul in the search for "America's next drag superstar." RuPaul plays the role of host, mentor, and head judge for this series, as contestants are given different challenges each week. RuPaul's Drag Race employs a panel of judges, including RuPaul, Michelle Visage, Ross Mathews, Carson Kressley, and a host of other guest judges, who critique contestants' progress throughout the competition.

Beginning with the seventh season, Michelle Visage, Ross Mathews and Carson Kressley are the staple judges alongside RuPaul. Visage joined the show at the start of season 3, while Mathews and Kressley joined at the start of season 7, and each joins RuPaul and Visage on alternate episodes. Past fixtures on the panel include Merle Ginsberg, who was a regular judge in the first two seasons, and Santino Rice, who held his position from the first season until the conclusion of the sixth. Until season 8, Rice was the only person, apart from RuPaul, to take part in every season of the show, serving as a main judge for seasons one through six, and all stars 1, and guest judging for season seven. In certain instances, Rice was absent and replacement judging has been provided by make-up artist Billy Brasfield (better known as Billy B), Mike Ruiz, Jeffrey Moran (Absolut Vodka marketing executive), or Lucian Piane. However, due to Brasfield's numerous appearances in seasons three and four, including appearing in the Reunited episodes both seasons, Rice and Billy B are considered to have been alternates for the same seat at the judges table throughout the two seasons.  Prior to the grande finale, the three main judges are joined by two celebrity guest judges each week. Guest judges have included Paula Abdul, Pamela Anderson, Eve, Ariana Grande, Neil Patrick Harris, Kathy Griffin, Debbie Harry, Khloe Kardashian, La Toya Jackson, Adam Lambert, Demi Lovato, Bob Mackie, Rose McGowan, Olivia Newton-John, Rebecca Romijn, Gigi Hadid, Sharon Osbourne, Dan Savage, John Waters, Michelle Williams, Candis Cayne, Martha Wash, Natalie Cole, Dita Von Teese, Niecy Nash, Debbie Reynolds, Vanessa Williams, Wilmer Valderrama, The Pointer Sisters, Trina, Leah Remini, The B-52's, Kesha and Lady Gaga.  The judges each provide their opinion on the contestants' performances in the main challenge and on the runway before RuPaul announces which queen is the episode's winner and which two had the weakest performances. The day before judging, the contestants are all provided with a song to which they must learn the lyrics. The contestants deemed as being the bottom two must "lip sync for their lives" to the song in a final attempt to impress RuPaul. After the lip sync, RuPaul alone decides who stays and who leaves. RuPaul describes the qualities the contestants must have to be crowned the winner of the show as "Charisma, Uniqueness, Nerve, and Talent... These are people who have taken adversity and turned it into something that is beautiful and something powerful." The phrase "charisma, uniqueness, nerve, and talent" is used repeatedly on the show, the acronym of which is CUNT. On the first All Stars season, "synergy" was added to provide an explanation behind the contestants being sorted into teams (expanding the acronym into CUNTS).

What do the judges judge?
The judges each provide their opinion on the contestants' performances in the main challenge