I realize this is an old thread, but I think the overall conclusion is misleading and so I thought the record should be set straight (particularly as "liguilla" is now starting for the Clausura 2007 in Mexico).
The following thread is much closer to correct:
http://forum.wordreference.com/showthread.php?t=250364
I believe that in Mexican soccer/futbol the terms "liguilla" and "repechaje" are closest to the American football terms of "playoff" and "wildcard," respectively.
In American football, if a team doesn't win their own division, they still have a chance to go to the playoffs as a "wildcard" team. This is based on the team's season win-loss record, compared across the whole league.
In Mexican soccer, there is a similar system. The first 2 teams from each group are in the playoffs automatically. But also other teams have a chance to qualify if their season record (in this case, points total) is good enough (specifically, if it is better than any of the top 2 teams in the other divisions). This is called "repechaje" - it is a form of "repesca" and is derived from the idea of trying to "fish" again - in other words, to get a second chance. (I think...Spanish-speaking natives correct me if I am wrong on that).
There are some differences - for example in American football, the number of "wildcard" positions is always the same. In Mexican soccer, the number of teams qualifying for repechaje is variable depending on the overall situation at the end of the season. But the idea is the same - it is a "second chance" to qualify for the playoffs, based on your record versus the overall league, even if you did not qualify by having a high enough position in your own division or group.
In American football, the "playoffs" are considered to start with the wildcard games. Thus, "wildcard" is more specific than "playoffs". That is, all wildcard games are considered to be playoff games; but only the first round of playoff games are wildcard games. I think the same should be true of "liguilla" and "repechaje", but I am not 100% sure. One could, for example, take the opposite point of view -- that the playoffs don't really start until the wildcard/repechaje round is finished. Some individuals in America may think that way too, but most do not -- especially those whose teams made it to the playoffs as a wildcard! This counts as "going to the playoffs" in their minds, for sure.
Incidentally, the use of the word "wildcard" in English is taken from the word for a card (naipes) that can take any value during play. In a similar manner, the "wildcard" playoff spots are not reserved in advance for any specific division, but can be filled by teams from any division depending on the season results. So the term "wildcard" in both cases means something that is unknown and is to be filled in or designated later.