Et cetera vs Et al

I would say there is a definite distinction between the two, in definition and in proper usage. Et cetera, often shortened to etc., means literally 'and the rest'. Et alii, often shortened to et al., means 'and others' and can be thought of as a specific case of et cetera when the 'rest' refers to a list of persons. It is often seen in academic contexts, usually when citing a reference having more than two or three authors.

I have seen improper usage of et cetera described as instances where what is intended to be the 'rest' is unclear or ambiguous, for example:

When visiting your accountant, please bring your receipts, cash book, bank statements, etc.

This would lead to confusion as to what the 'etc' represents. What other papers would we need to take to the accountant?

A more correct usage may be:

The mini bar is stocked with a wide range of spirits: vodka, gin, tequila, etc.

In this case there is no confusion as to the intended meaning of the sentence: the mini bar is stocked with a variety of alcohol.

It's the context of those other things that makes the difference here. Et cetera means "and the rest" and et al. means "and the others". Specifically, et al. stands for either et alii, et aliae or et alia when referring to masculine, feminine or gender neutral groups respectively.

Et al. should be used when referring to groups of people, not things. You'll commonly see it when authors are cited in academic works.

Also interesting is that et al. can also be an abbreviation of et alibi, which means all the other places, although this variant is apparently less common.

Typically, etc. refers to things and et al. to people but I don't think that's the key distinction.

Etc. means "others of the same sort". Et al. means "other from the same group" or "others forming some group". So "mice, rats, etc." but "Snooki, Sitch, et al."

The reader should always be able to fill in more (plausible if not necessarily correct) examples with "etc." so "mice, rats, voles, gophers..." (on the assumption that the "sort" being enumerated is rodents), whereas my et al. example refers to a fixed set (fluorescent orange cast members of a particular TV show) where there are other members but I can't name them.