Meaning:

• Women and children use の with a low intonation to emphasizes an explanation and to solicit familiar empathy.
鉛筆がなくなったの。I lost my pencil.
今日は、試験があるの。It's that I have a test today.
 
• When a man, woman, or a child uses it with a rising pitch, の turns the sentence into a question.
鉛筆がなくなったの? Did you lose your pencil?
これから寄るところがあるの? Is there somewhere you are stopping by after this?
 

Location:  Behind V-plain, い-Adj-plain, な-Adj + な or copula, N + な or copula.

Gender:  The falling tone is used by both men and women but the rising tone is used predominantly by women.

Level:  Informal

Warning:

※Only adult women would use の behind the formal form of the copula (です／でした／etc.).
        明日行くんですの。(Someone) is going tomorrow.

※Click here to see particle no (の) page.