This started in 2000, and it became very popular, though of course I haven't seen much of it. (As of 2014, it's still producing new episodes.) It's for preschoolers or maybe very early grade schoolers. There are shows for that age group that I like, but this isn't one of them. I mean, I don't have anything particularly against it, except that it's fairly condescending to the viewer. (Not that it acts superior to kids who might be watching, it just seems like even kids in the target age group should be way smarter than the show gives them credit for, simply because the characters themselves, while very cheerful and friendly, are actually pretty incompetent.) It's hard for me to imagine anyone actually learns anything from it. Not that I'm entirely sure what it's even trying to teach.
Anyway, Dora is this 7-year-old Latina, who mixes a bit of Spanish into the mostly English that she speaks. She has friends (mostly animals I think, mainly a monkey named Boots, but there are also anthropomorphized objects like Dora's backpack and map). And they go on adventures, look for stuff, and ask the audience to help them, or whatever. I can't really tell you any more than that, except that every once in a great while, it might be vaguely fun to tune in and watch it ironically, pretend your intellect is actually on the level the characters expect it to be. (In fact I wouldn't be surprised if some teens or young adults enjoy this kind of thing a lot more than I ever would, particularly if they're high or something.) Incidentally, I also remember one year (probably 2004) seeing a Halloween DVD in stores (I've never actually watched the special), in which Dora was dressed as a catgirl, which made me joke on a message board I went to that I was upset that I was now forced to find Dora hot (because of my love for anime girls in general, including catgirls). That was, of course, not even a little bit true, which is what makes it potentially mildly amusing (but probably not).
Oh yeah, I should also mention there was a spin-off series called "Go, Diego, Go!" (2005-2011), about Dora's cousin. But I'm pretty sure I've never even seen any of that. In 2014, another spin-off began, "Dora and Friends: Into the City." In it, Dora is ten years old and living in a city, instead of exploring jungles and stuff. But at the time it started I didn't have access to Nickelodeon, so I haven't had a chance to check it out.
In 2013, I happened to see a fake movie trailer on YouTube, for a live-action action/adventure movie about Dora, starring Ariel Winter, and that's what prompted me to finally start an entry for the show. (I'd considered it numerous times before, but never got around to it.) I think if you've ever seen even a few minutes of the show, you should find the trailer hilarious and awesome. The trailer originally appeared on College Humor, and it was so popular that they eventually made an actual miniseries based on it, for the website. Speaking of parodies, there was also a segment on MAD that was a mashup of Dora and The Legend of Korra." (It's from 2012, but I first saw it in 2015.) That was pretty funny. And in 2019, a real live-action movie called Dora and the Lost City of Gold was released.