I've looked in the FAQ regarding search, specifically the following part:
- To search within specific tag(s), enter them in square brackets: [maintenance] seat.
- To find a specific phrase, enter it in quotes: "flat tire".
- To ensure that specific words appear in the results, use plus: +frame +steel.
- To search just your posts: user:me training.
So the plus operator ensures that words appear in the results. If you don't use the plus operator, what does it do? Isn't the point of the search to find all words?
EDIT: This question is no longer significant. As of 2013, StackOverflow uses an AND-based search:
The new elasticsearch-backed search is using an
ANDoperator by default now.