Here's a quick guide for those who aren't in court everyday:
Sustained: When an objection is sustained, the judge has determined that is a valid objection. That means the question was improper under the rules of evidence. The witness may not answer the question. (If the witness answers anyway, that answer may be "stricken.")
An easy way to remember this is Sustained = Stop, as in the witness must stop and not answer the lawyer's prior question.
Overruled: When an objection is overruled, the judge has determined the objection is invalid. The question may stand. The witness must then answer the question.
A shorthand way to remember this is Overruled = Ongoing, as in the witness may continue as if the objection never occurred. Hope this helps!
For more information about some of the differences between the way things happen in fictionalized legal proceedings and in real life, get my free Writer's Guide: Top 7 Mistakes Made By Writers of Mystery, Crime and Legal Drama.