A limiter is a way to narrow the scope of your search. For example, instead of sifting through thousands of articles published up until 2012, you can limit your search to find articles published only between 2000 and 2012.
An expander is a way to broaden the scope of your search. For example, instead of searching one term that may restrict your results, you can expand your search by selecting the Apply Related Words option. This will include synonyms and plurals of your term in the search and return more results.