There are four Latin abbreviations that many people confuse and use incorrectly. (Don't even get me started on those who say or write ect. instead of etc.—a teacher I had in high school did this all the time and it drove me nutso!)
I thought it would be smart to explain what each abbreviation means in Latin and give an example, just so we know we're using each correctly, and in the right context.
Let's start with the more obscure of the four, N.B.
N.B. is the abbreviation for the Latin nota bene, which means note well. Use N.B. (or NB:) when you want someone to pay close attention to a particular sentence, caveat, parenthetical, etc. For example, NB: The views expressed in this writer's post are not necessarily shared or endorsed by mentalfloss.com.
2. et al.
Et al. stands for et alia in Latin, or and others. Use it when you want to shorten a list of names. So if we were talking about all the bloggers who write for mentalfloss, we might write, Jason, Mangesh, Sandy Ransom, Stacy et al.
In Latin, i.e. stands for id est, or that is. Use it in place of in other words when you want to clarify what you just wrote/said. For example: There are many blogs out there, but this one (i.e. mentalfloss.com) is far and away the most informative.
Exempli gratia in Latin means for example. You want to use e.g. when you're naming a specific instance, as in: I've enjoyed many of Chris Higgins' posts on this blog (e.g. Daguerreotype Q&A, Old Computer Ads)