Buddhism and fornication

Most of the people follow the 3rd precept of lay buddhism without knowing the actual meaning of it. Personally, even I don't know the actual meaning of it. Most of the people including me understand it as, "not practicing adultery outside marriage". But I know there's more than that. I simply don't know what. Can you give a clear answer for this two questions. - What is the actual meaning of the 3rd precept? - What does it(the 3rd precept) say about "fornication"?