"Not guilty" and "acquittal" are synonymous.
A verdict of not guilty constitutes an acquittal. In other words, to find a defendant not guilty is to acquit. At trial, an acquittal occurs when the jury (or the judge if it's a judge trial) determines that the prosecution hasn't proved the defendant guilty beyond a reasonable doubt. (But see Jury Nullification.)
A jury can find a defendant not guilty of some, but not all charges. In that scenario, the acquittal is only partial.
A not-guilty verdict isn't the only way for an acquittal to come about. Trial judges and appeals courts can, for example, effectively acquit defendants by finding that there was insufficient evidence of guilt. While there's no way for the prosecution to appeal a verdict of not guilty, there is sometimes an opportunity to appeal a court's judgment of acquittal. (For example, see Acquittals by Judges in Jury Trials.)
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