This is more than just a book series, Harry Potter is a "fandom" (a fan kingdom, very popular among amazing book series) that is across the world. When J.K. Rowling was writing about a skinny wizard with an extraordinary past, living in a slave type environment, I don't think she every dreamed it would get this huge.
There are 7 books in the series. The follow Harry Potter, a young boy raised by his muggle (non-magic) Aunt and Uncle. He is about the same age as his spoiled rotten cousin Dudley, who is also a bully. When his Aunt and Uncle "decided" to take Harry in, they said they would never let him know about his past, his parents or anything in the wizarding world. His parents were killed by the most evil of wizards, but he believes they died in a car crash. His Aunt and Uncle hate anything to do with wizards and magic. They forbid any use of the word magic, and try to hide any signs of it. Anytime Harry used magic by accident, he was punished (though he didn't know why) and it was played off. Not the nicest of people, his family was.
Of course, the magic world couldn't be hidden from him forever. Every witch or wizard, after their 11th birthday, would receive a letter to be admitted to a wizarding school. Harry was no different. His uncle kept trying to hide the letters, and after much trouble, Harry was located and told the truth. He did, indeed, go to the school, where he met his 2 best friends.
Over the series, things get sticky, rocky and sometimes just confusing. The adventures, creatures, classes, it's all amazing. J.K. Rowling did an amazing job, and so many people all over the world have fallen in love with this. From being 7 wonderful, amazing books, came 8 movies, and a variety of other things. Tattoos, websites, friendships all over. People are in love.
Now, before you think this is all a bunch of hocus pocus, and mixing bat brains, it's not. This series teaches you (and your children) great values. Standing up for your beliefs, friends and family. True honor and friendship, bravery. That you may have it bad, but someone has it worse. The value of life, and people. It's entertaining and educational, in all the things that count.
One of the best websites out there is Pottermore.com. Made by Jo Rowling for us fans. It helps us reach inside her head, and see things how she saw them, wrote them. It goes book by book, chapter by chapter. You can collect items from each book, and you can even play games, like potions class. I personally suck at this potions class, but its fun to play!
There is also Mugglenet.com. Made by fans, you can find almost anything, everything potter related. There are links for recipes, games, costumes. It's one of a kind.
Myhogwarts.co.uk is still in beta, I belive. I am in membership, but so far they have changed the site like 3 times, that I have noticed. I personally loved the original way they had it, design and all. I met someone in my "dorm" that I am still great friends with! I love this girl, and one day hope to meet her. We have so much more in common than just Harry Potter, true nerdy friends.
Then you have the games. All sorts of games, scene it (I own it!), card games (I own UNO HP style!) and video games galore! When the Game Cube was huge, it had all kinds of games. I own them all, even though I don't play them due to lack of proper chords. When we first got our PS3, before we had to sell it... Hard times suck! I had the Lego HP game. I loved it, and now that we have another PS3, I am hoping to acquire this game!
Harry Potter inspired tattoos are now all the rage. I hope to get one or 2 myself, though I am having such a hard time trying to decided the 1 or 2 I like the best!
The fandom has even inspired a theme park! I one day hope to go. It's said to be amazing, and I want to be there. To ride the rides, taste the food, see EVERYTHING! I am a potterhead, through and through. I have JK Rowling to thank for my childhood. It was amazing because of her.