Pure curiosity, I see that Mel Gibson is an adherent of Traditionalist Catholicism. Is this a part of "normal" Roman Catholicism, or some distinct branch? What are the differentiators here?
I found this information hidden at the bottom of your link:
Mel Gibson belongs to a Catholic sect that does not accept the reforms of the Second Vatican Council. - Time, January 27, 2003
After some more digging, I found that the main differences are:
They still practice Tridentine Mass. This mass has a set liturgy which you can look at more closely in the above link. The most immediately distinguishable feature of tridentine mass is that it is performed almost exclusively in latin.
They hold onto some of the of the personal customs that were largely depreciated after the Second Vatican Counsel. The ones that struck me the most were:
- Abstaining from meat on Fridays,
- Fasting from midnight before receiving Holy Communion, and
- Women wearing a headcovering in church.
It is worthy to note that the Vatican usually supports traditionalists, unless of course those traditionalists condemn modern day leadership or practices.
Most of this information was found on Wikipedia, I just tried to summarize it. There's plenty more information if you want to take a look!
EDIT (In response to a good comment by @Alypius): Here's a description of the different types of traditionalists, as described by an actual traditionalist. Religious preferences aside (I don't agree with the main gist of the article), the category description quoted below seems to be pretty succinct.
"The first and by far the largest group consists of those Catholics who accept the acclaimed Pope and his recent predecessors as true Popes and who believe that the Second Vatican Council was a valid, albeit problematic, Council.
The second group consists of those who are unsure about the status of the acclaimed Pope. Many such Catholics worship at Masses offered by the Society of Saint Pius V (S.S.P.V.).
The third group consists of "sedevacantist" Catholics, that is Catholics who believe that the Catholic Church has not had a true Pope for some time (most consider Pope Pius XII as the last true Pope) and who, depending on the time they see as the moment the "Chair of Peter" (sede) became empty (vacante), may or may not see Vatican II as a valid Council. Many sedevacantists attend Masses offered by the Congregation of Mary Immaculate Queen (C.M.R.I.). 5"
One must note that there is no single "Traditionalist Sect/Branch". The word "traditionalist catholic" may refer to anyone from someone who prefers Tridentine Mass over NOM to sedevacantists and followers of antipopes.
Some traditional catholics live in full communion with catholic church. They might be sceptical about SV II. They might disagree with some modernist tendencies.
The brotherhood of Saint Pius X is one of the biggest organisation of traditional catholics. It never rejected the pope, although during the pontificate of JP II it was excommunicated for consecrating bishops without pope's order. The unity was brought back by Pope Benedict XVI.
"Sedevacantist" is a term that generally describes someone that believes there is no pope. There is no single sedevacantist group and sedevacantists differ in the opinion which was the first unlawful pope.
Among the current antipopes there are also some that could be labeled as "traditionalists".