All words returns only documents that have all of the keywords you enter ("AND" search).
Any words returns documents that have any of the keywords you enter (i.e., "OR" search).
Substring returns documents that contain words with the substring in them (e.g., a substring search on "think" will return documents with the keywords "think," "unthinkable," "thinktank" etc.)
Exact phrase returns only documents that have the exact phrase you entered.
Fuzzy search will return results that are "close" matches to the keywords you enter, often correcting common spelling errors.
Topic search returns results that include keywords that are topically related to your keyword (e.g., a search on "sport" will return documents that include the word "baseball.").