Like the particle か (KA), the particle の (NO) can be placed at the end of a sentence to indicate that it's a question in an informal way.
いつスキーに行くの?
Itsu sukii ni iku no?
When are you going skiing?
2.
The particle の (NO) can be used at the end of a sentence to build an imperative (order). Depending on the intonation, the order can be quite direct or soft.
そんな目で見ないの!
Sonna me de minai no!
Don’t look like that!
3.
Women sometimes use the particle の (NO) to soften a statement.
毎週日曜日はヨガの教室に行くの。
Maishuu nichiyoubi wa yoga no kyoushitsu ni iku no.