Search engines use statistics and probability to match documents to your query and rank their degree of relevance. The more often your terms appear in the document, the more relevant that document is considered. The length of the document can affect its relevancy ranking. When terms appear at the beginning of a document, many search engines assign that document more relevancy. Relevancy is also assigned by how close multiple terms appear together.
Most search engines provide automatic stemming, truncation, and synonyms.
They also have stop words, which means they eliminate very common terms from your search. Most display the most relevant results on top. Results frequently include exact and fuzzy matches. Fuzzy matches may have two of your terms, but not the third. These partial matches broaden your search.