If you're wondering what the point of Instagram is, it's to take a photos and share them very quickly. You open the app, take the photo, write a very short caption, and share. I just started doing it a few days ago and it is really, really, fast.
You can, of course, make it more difficult if you'd like. You can add filters to your photos if you want to (although that trend is going away - the idea being to use a filter to make your photo look like an old Polaroid from the 1970s), but that's not really what makes the app so popular.
Sure, you can do this kind of stuff on other social media sites, but nothing goes as quickly from *click* on your smart phone to sharing. And, oh yeah, you need a smart phone for this. You don't post from the web on Instagram. You take photos.
Now be careful here. Just like tweeting, think about what you are saying, and showing. Instagram is a potentially embarrassing place to be if you don't know web etiquette. Stick with something safer, like Facebook, if you're not sure how all of that works. But if you're comfortable with posting things on the web, and you can manage not to show or say something that's embarrassing, Instagram is a lot of fun.