You’ll get asked about these, too – how and when to use them, what they stand for, and why they exist.
et al.: This is the abbreviate form of et alii (“and others”)—the others being people, not things. Since al., is an abbreviation, the period is required.
etc.: This is the abbreviated form of et cetera (“and other things”); it should never be used in reference to people. Etc. implies that a list of things is too extensive to recite. But often writers seem to run out of thoughts and seem to tack on etc. for no real purpose. Also, two redundancies often appear with this abbreviation: 1) and etc., which is poor style because et means “and,” and 2) etc. at the end of a list that begins with e.g., which properly introduces a short list of examples.
i.e. and e.g.: The former is the abbreviation for id est (“that is”), and latter is the abbreviation for exemplia gratia (“for example”). The English equivalents are preferable in formal prose, though sometimes the quickness of these two-letter abbreviations makes them desirable. Always use periods and put a comma after either of them.
[sic]: You may be reading an essay or a text for a class when you run across this little guy [sic.] Sic provides information about the text you’re reading. It only appears when the author is quoting another source. And it only appears after words that are either misspelled or used incorrectly. Sic always appears between brackets – never in parentheses. Sic has been around a long time – since the time of the Romans, actually.
The purpose of sic it to tell you that the author of what you are reading is not a complete idiot but is simply providing an accurate citation. Sic is translated as “thus in he original”; it lets you know that the mistake is part of the text being quoted and not part of the text you are reading.