There is a good deal of confusion surrounding et al., as is commonly the case with abbreviated Latin words (such as ibid., etc., and i.e.).
Et al. comes from the Latin phrase meaning “and others.” It is usually styled with a period, but you will occasionally see et al as well.
Et al. typically stands in for two or more names, especially in bibliographical information. It's preceded by a comma only when more than one name is listed (as in cases in which two or more texts are authored by the same person or by people with the same surname):
The book by Carson et al. is regarded as the authoritative text on the topic.
The article by Jones, Perez, et al. is well-known, but the one by Jones, Lee, et al. has been more widely cited.
It is only followed by a comma in cases in which the phrase "and others" would be followed by a comma:
The documentation provided, credited to Hadid et al., is sufficient for our purposes.
The documentation Hadid et al. have provided is sufficient for our purposes.
Punctuation isn't the only confusing thing about et al. Its Latin origin causes some confusion too, because the phrase in Latin could be written three different ways, depending on whether the other things one referred to were masculine (et alii), feminine (et aliae), or neuter (et alia).
Another aspect of this word that creates confusion is the question of when it is appropriate to use it. Some language guides have argued that et al. should only be used to describe people. However, it is used (albeit infrequently) in reference to other things as well.
Et al. is most commonly found in scholarly writing, especially when used to avoid having to list a number of different authors in a bibliography or footnote. You can use it when describing the people who came to a dinner party, but it may sound rather odd. Some of the Latin abbreviations found in English have become well-suited to conversational usage (we often hear i.e. used in speech), while others appear out of place. For instance, few people would say "ibid." (which means "in the same place") in response to the question "where are my hat and gloves?"
History and Etymology for et al.
Latin et alii (masculine), et aliae (feminine), or et alia (neuter)