13. A computer-implemented method comprising: receiving an order including keywords for an online advertising system, wherein individual keywords include at least two words comprising a first word and a second word; breaking the keywords into the at least two words; forming a word list from the at least two words by sorting the at least two words decreasingly by frequency of occurrence in the order; determining that correcting the first word to the second word meets misspelling candidate criteria, the misspelling candidate criteria including: a frequency of occurrence of the second word is higher than a frequency of occurrence of the first word, stemming forms of the first word and the second word are not the same, an edit distance from the first word to the second word is less than a first threshold, a ratio of the length of the first word to the edit distance is larger than or equal to a second threshold, and the frequency of occurrence of the first word is less than a third threshold; in an instance when the misspelling candidate criteria are met, computing a misspelling confidence score and correcting spelling of the first word using the second word when the misspelling confidence score exceeds a fourth threshold; and outputting corrected keywords that include the corrected first word.
13. A computer-implemented method comprising: receiving an order including keywords for an online advertising system, wherein individual keywords include at least two words comprising a first word and a second word; breaking the keywords into the at least two words; forming a word list from the at least two words by sorting the at least two words decreasingly by frequency of occurrence in the order; determining that correcting the first word to the second word meets misspelling candidate criteria, the misspelling candidate criteria including: a frequency of occurrence of the second word is higher than a frequency of occurrence of the first word, stemming forms of the first word and the second word are not the same, an edit distance from the first word to the second word is less than a first threshold, a ratio of the length of the first word to the edit distance is larger than or equal to a second threshold, and the frequency of occurrence of the first word is less than a third threshold; in an instance when the misspelling candidate criteria are met, computing a misspelling confidence score and correcting spelling of the first word using the second word when the misspelling confidence score exceeds a fourth threshold; and outputting corrected keywords that include the corrected first word. 16. The method of claim 13 , further comprising: determining that a third word in the word list is a sub-word of fourth word in the word list; and correcting the third word by spelling the third word like the fourth word.