The suffering passive is a term used to describe a usage of the passive form of Japanese verbs that indicates distress, suffering and so on. It may seem a little odd to native English speakers, but once you get used to it, it's not so bad.
In most case, when you use the passive forms of verbs in Japanese, you mark the subject with ga and the actor with ni, right? For example, if you want to say "my diary was read by my mother", you would end up with something to the effect of "nikki ga haha ni yomareta".
So what if you run into "haha ni nikki wo yomareta"? What the hell is that wo doing? And where's the subject marked by ga?
Well, if you were to force the subject in, you'd get "watashi ga haha ni nikki wo yomareta". This is a suffering passive, and when you use the suffering passive and there isn't a subject obvious from context, then most likely you're moaning about something that's happened to you. What about the rest, then? Well, your nosy mother is still the awful person who's done this act to you, so she gets to keep her ni. Finally, your diary takes wo because it is too busy being the direct object of the act of reading to be the subject of the sentence, which as you recall, is (probably) already watashi anyway.
What's the point of doing all this, then? Well, rather than meaning just "my diary was read by my mother", it has
acquired an emotional emphasis. It becomes something to the effect of "Damn it! My mother read my diary!", which is decidedly much better for moaning about this thing that's been inflicted on you than just blandly stating what happened.
You may also notice that when translated into English, the sentence is not passive. This occurs as the original sentence is in fact no longer strictly speaking passive, because it has a direct object.
Additionally, suffering passives are often accompanied by verbs such as shimau and komaru for emphasis. Since in the real world, sentences don't just float around in nothingness, often there will be other indications that the speaker is moaning about something that happened to them.
Some more examples:
Ame ni furarete komatta = Curses, I got rained on!
Inu ni te wo kamarete shimatta = That damn dog bit my hand!
Saifu wo nusumareta = Oh no, they stole my wallet!
I wrote this up just because I love the term "suffering passive". Teeheehee.