I hate definitions, but I'll try. Nobody seems to have problem to recognize 3rd and 1st-person narratives, but 2nd-person seems to be a hard nut to crack. Even my copy of Concise Oxford Dictionary of Literary Terms doesn't take its existence into consideration. So poor Helen is left to her own limited devices... And my very layman explanation would be:

• In 1st-person narrative the narrator is one of the actors of the story, whether the main or supporting character, and he, she, or it (yep, I've read an excellent story from the POV of a cat :) is referred to as "I".
  Example: Magpie Poet's Too Smart (this is the cat story I've mentioned).
• In 3rd-person narrative the narrator is outside the events described, not one of the characters. The characters are referred to as "he", "she" or "it" (or by their names, of course). This kind of narrator may be "omniscient" (all-knowing) or "limited" (restricted to the POV of only one character).
  Example: Lord of the Rings is written in 3rd-person omniscient; Harry Potter novels are written in 3rd-person limited.
• In 2nd-person narrative there is no narrator, the main character is referred to as "you".
  Example: Stories archived at this website. :D
• Note 1. Some 1st-person narratives are told as if addressing another person, and thus may sound like 2nd-person narratives. But they still have the "I" narrator, and whatever literary theory says on this point, I don't consider these as 2nd-person narratives for the purpose of this website (but I may change my mind). I call this device "fake 2nd-person".
  Example: Eilan's Silence.
• Note 2. In some stories, the narrator tends to address the reader directly as "you", but that again is not 2nd-person narrative (it's 3rd-person) and I absolutely hate this device. It's suited for children's books, and I find it annoying even there.
  Example: The Hobbit was partly written this way, but even Tolkien himself admitted later that he found this annoying. :)
• Note 3. In the discussions on 2nd-person, some people mentioned something called "Choose Your Own Adventures" books. Puh-leeease, do not even consider to suggest that THIS is 2nd-person narrative or I may grow prematurely grey-haired. :)