Harry Potter is a series of seven fantasy books written by J.K. Rowling, whose plot can be summed up as:
An Evil Overlord that was thought to be long gone is coming back. The Chosen One must defeat him by embarking on a epic quest to destroy magical objcets related to said Evil Overlord - objects that reveal ties between Our Hero and the antagonist. He has the assistance of a wise old Wizard with a long grey beard, that will leave him along the journey. Yes, you've seen it before
It's a much beloved young adult fantasy series that started as a story for kids and kinda grew in tone along with the age of the audience. Yes, you've seen that before, too
Harry Potter is basically the forefather of the current young adult Urban fantasy genre and series like Percy Jackson and the Olympians owe more than a small intellectual debt to it. While things like Anne Rice's novels and Vampire the Masquerade may have brought the Urban fantasy genre into being in a recognizable format (well, disregarding western comic books which are either considered their own genre or a kind of urban fantasy depending on who you ask) it was Harry Potter that brought the genre to kids. Essentially any fictional series about a kid from the ordinary world being whisked away into a secretive mystical one to face mystical problems as well as the issues of it being hard to be a kid growing up made from the 90s onwards owes something to Harry Potter even if it's a story about deconstructing the Harry Potter type narrative. It also showed that there was a huge amount of money to be made from writing for tweens and teenagers specifically instead of choosing to go for either young children or adults. We're talking "quite possibly the most profitable demographic to market towards" here.
In spite of this, no book has gotten even close to as much popularity among kids as Harry Potter. It was believed at the time that the series would bring about a revolution in getting kids to actually read, but very few kids actually picked up any other books once the series ended (and instead of expanding those kids horizons as it was widely believed reading would do, many of these kids would grow up to be unable to discuss serious issues without relating it in some way to Harry Potter - it's an embarrassingly common phenomenon at protests these days). The closest to recapturing that kind of magic was Percy Jackson which was hobbled by some amazingly poorly thought out movie adaptations, though the Percy Jackson fandom is still very much on the large side for a fandom. However you could also argue that this is because after Harry Potter, when the young adult urban fantasy genre took off with a bunch more writers getting into it the readership also fragmented into a bunch of other series. Much like how no other space fantasy series has ever really managed to get even close to Star Wars level popular, and no Gothic Space Fantasy series has even approached 40k's popularity. The first in the genre to really take off tends to get the benefit of having no real competition when it first grows, while everything following it will have to fight a bunch of other people who also want to ride that wave.
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The story is about an orphaned boy living with awful relatives. He soon finds out he has magical abilites and goes to a Wizard boarding school where he makes friends, learns magic and does magical sports. Soon enough, learns about his family and gets wrapped up in affairs involving a Dark Wizard version of Hitler called Lord Voldemort and his associated assholes (including a Dark Wizard version of the Klan called Death Eaters and Nazgul rip-offs called Dementors). So basically the pipe dream of every disaffected teenager; this more than anything probably explains the series' breakout popularity.
The books sold really well, got a series of very popular movies which grossed higher than any other series of movies in history, probably got a fair bit of people interested in fantasy literature (given that they were mainly targeted to young adult readers) and generated a moderate amount of skub back in its day before the haters moved onto things which were more uniformly panned. Given the target audience, it was also inevitable that the fandom created an unholy amount of fanfiction, including what's universally recognized as the worst fanfiction ever. But it is also the source of the best (and most batshit insane) fanfiction ever, as well.
In general the series has good characters, even though the main cast looks a bit lacklustre when you think about them, and the main antagonist has not much to him besides "I'm Hitler, but with magic".
The lore and world-building is at best hit and miss, and sometimes you feel that the author is pulling deus ex machinas and lore out of her ass to railroad the story forward, but the series is not the worst gateway drug to the world of fantasy literature a young kid could have, even if traditionalists would favor Tolkien, and of course many a writefag would argue that worldbuilding and lore are secondary at best to a consistent theme, plot, and good characterization. Similarly, as what is essentially the forefather of teenager oriented urban fantasy; it's obviously going to have the usual issues that plague other series that basically spawn a subgenre around them; other later series can learn from it and build on it. Much like how Seinfeld doesn't seem all that special today because its lessons have been so thoroughly disseminated throughout the genre that looking at Seinfeld now is like looking at a prototype of a line of products you're already familiar with. That being said despite quite a lot of competition (the most serious being Percy Jackson though the fandoms themselves are on good terms), Harry Potter still generally holds up as one of the better examples of young adult urban fantasy literature.
Eh, no reason you can't try both Tolkien and Rowling.
- Harry Potter: The Boy Who Lived and main protagonist. An unassuming English kid with glasses that obtains a pet owl, and takes up his preordained destiny to enter a secret world of magic hidden in plain sight. The Dark Lord tried to kill him when he was a toddler, but his parents loved him and the spell bounced and made the Dark Lord vanish instead (if that raises questions you've probably already put more thought into it than the author did). Went to stay with his abusive aunt and uncle and didn't notice he was a Wizard until a hobo came to his house and told him. Not the smartest knife in the drawer, and for much of the series he's actually more hated than loved by the wizardry world due to him being an angsty kid and the author catering to the needs of his angsty kid audicence.
- Hermione Granger: Smart nerd girl and probably your first erection. When she gets a magical object that allows her to travel through time she uses it to study more instead of, for example, solving every problem ever. Out of the blue she decided to bone the Comic Relief character at the end of the last book despite treating him as a dimwit for 7 years. The author has admitted this was a mistake.
- Ron Weasley: Redhead comic relief. That's about it. Once he had a pet rat that was an old hairy man in disguise and slept with him. His brothers, due to the Marauder's Map (a magical object that shows the location of everybody in Hogwarts, with all the unfortunate implications), probably knew about this and was totally ok with it. Bangs a chick way out of his league due to contrived plot reasons.
- Voldemort: aka Magic Hitler. No, really. He wants to eliminate everyone with muggle ancestry, wizard or not. Why? Because his mother date-raped his father with a love potion, said father abandoned them after getting off the potion and she died giving birth to him. That sucks, but no need to take out your issues on the rest of mankind. For half of the series he's in a ghostlike state until he gets himself a new, strangely noseless body (he was noseless before, it was a side-effect of splitting his soul and putting the pieces in soul jars), thanks to the fact that he split his soul up into a bunch of different objects. Is finally killed for real when Harry destroys all the Horucruxes - himself included. WHAT A TWIST! But then Harry's still alive because he's the master of the Deathly Hollows! DOUBLE TWEEST!...yeah.