Let's get this out of the way first: yes, it's a TV show that's primarily aimed at young girls. My Little Pony: Friendship is Magic is a children's animated cartoon for Hasbro's My Little Pony franchise that has become wildly popular, both with kids and, surprisingly, The Internet.
Somehow a cartoon about cute little multi-colored ponies has captured the interest of thousands around the world. For some it's because it's well animated, for others it's because it's a funny, well-written show that entertains both kids and adults. The end result has created a massive fandom following that's arranged multiple large conventions, a huge collection of fanart and fanfic, has a thriving remix music fandom in and of itself, and fans creating pony mods for hundreds of games and pretty much everything else in between.
And all this around a show that has singing ponies and magical beams of friendship. I'm not making this up.
I'm not going to try to convince you to "join the herd"; if you're reading this you've probably already got an opinion of MLP. If, however, you've managed to avoid the pony phenomenon and are wondering what this crazy pony craze is, the easiest thing is to watch a few episodes yourself - luckily the Internet Will Always Provide.