Putting double quotation marks around search words makes the reference engine find exact matches. The question mark is provided especially for crossword junkies. Use a question mark for each unknown letter and the reference engine will find all possible words that match. These words are known as stop words and can cause problems for some search engines. But at xrefer they are only indexed in headings, so you can search for a dictionary entry on the word and without having to worry about finding every entry which contains the word and.
Capital letters have no effect on searches.
It can be software, online services, you name it - but it's got to be free, with no strings attached. I've yet to see much good actually said about it. We don't sell links in default bookmarks, ratings in search engines or similar. We believe in open competition on a distributed Internet. A big plus for many.