* An asterisk (*) stands for any number of characters, including none, and is especially useful when you want to find all words that share the same root. For example, pigment* matches pigment, pigments, pigmentation, etc. Note that you must enter at least three (3) non-wildcard characters. So, a search on o* is not allowed; rather you need to enter oba*. An asterisk can also be used within a word, but the other wildcards are more precise for this kind of use.
? A question mark (?) stands for exactly one character and is especially useful when you're uncertain of a spelling. For example, a search like relev?nce means you can match the word relevance, even if, like many of us, you can't remember whether it's spelled with ance or ence. A question mark is also useful for finding certain words with varian spellings. For example, defen?e finds both defense (American) and defence (British and Canadian). Multiple question marks in a row stand for the same number of characters as there are question marks. For example, psych????y matches either psychology or psychiatry but not psychotherapy.
! An exclamation point (!) stands for one or no characters and is especially useful when you want to match the singular and plural of a word, but not other forms. For example, product! matches product and products but not productive or productivity. The exclamation point can also be used inside a word to match certain variant spellings. For example, colo!r matches both color (American) and colour (British).