The search also finds parts of words, so if you search for petter you will also find petter, pettersson and petters. Read under "exact words" below how to avoid this. Upper/lower case is not important, so PETTER, petter and Petter will all find the same texts.
Normally all pages where all your search words appear will be shown. You don't have to write + between the words. If you for example search for alfred pettersson you will only find pages where both these words appear.
To search for a phrase, enclose it in quotes " ". If you for example search for "Anders Larsson" you will only find pages containing this exact name, but not those where both words occur but separated, e.g. "Niklas Larsson och Nils Andersson".
You can also use a minus sign - before a word to exclude it from the search. If you for example search for "bruun -lorenz" you will find pages where the name bruun occurs, but exclude those where the name lorenz occurs.
To search for whole words, not parts of words, you can use an equal sign = before the word. If you for example want to find Ersson but not Andersson and Pettersson you write =Ersson in the search box.
If you're not sure how a name, place or title is spelled you can use approximate search. Then you will find "near matches" for your spelling. If you for example want to find a Lundkvist but don't know how it was spelled, approximate search will find Lundkvist, Lundqvist and Lundquist (but also Sundkvist och Lindkvist, which also differ by just one letter).
Also take a look at the search tips.