んだって

When used at the end of sentences, だって often expresses something that has been heard by the speaker. This particular use of だって will always be paired with ん or なん.
As だって is a grouping of the auxiliary verb だ, and the adverbial particle って, なんだって primarily expresses the following:
なん - Presents an explanation.
だ - Determines something to be true.
って - An abbreviation of と言(い)って 'is said' or と思(おも)って 'is thought'.
In this way, it translates quite similarly to 'it is that (A) was said/thought'. Realistically though, it is used in the same way as 'I heard that (A)', or 'it's thought that (A)'.

• サクラちゃん来年(らいねん)結婚(けっこん)するんだって。うらやましいね。
  I heard that Sakura-chan is going to get married next year. I am jealous, aren't you?
• タケル君(くん)が新(あたら)しいおもちゃが欲(ほし)しいんだって。どうしよう？買(か)ってあげる？
  I heard that Takeru-kun is wanting a new toy. What should we do? Should we buy it for him?
• あの博士(はかせ)は凄(すご)い事(こと)言(い)っているように聞こえるけど、実(じつ)は言(い)っていること全部(ぜんぶ)適当(てきとう)なんだって。
  It sounds like that scientist is saying some amazing stuff, but I heard that everything he says is hot air.
• タクミさんはレスラーみたいな体(からだ)しているけど、好(す)きなスポーツはフィギュアスケートなんだって。
  Takumi-san has a body like a wrestler, but I heard that his favorite sport is figure skating.