Instagram is a social photo-sharing service. You post photos and people that "follow" you can see them. It's really that simple. Once you download the app for your device (iPhone app, Android app, and Windows app) and sign up for an account, you are all set to share.
Instagram is a free service and the associated apps are also free. Instagram (owned by Facebook) has talked about inserted ads into your list of photos (not into your actual photos, however), but so far that hasn't happened yet.
Instagram is a great way to share photos without having to email them to family and friends. And because it's just a photo, it takes practically no time to look at several (or several dozen).
Yes, you can set your account to private. And only the people you approve can see your photos.
The Heart Button
That's a way to "like" a photo. You tap it (or you can double-tap the photo itself) and that tells the person you liked it.
Available Filters and Editing Tools
One of the fun things that Instagram introduced was a way to give a little extra personality to your photos by adding filters that can make the picture look older, or make it black and white, or even sepia (that's a small sample of the filters).
Instagram updates the photos from time to time, too. Some photo purists say the filters ruin the photos, but that seems a little bah humbug since 1) they're optional and 2) have some fun, would you? The editing tools can help you fix a tilt (for when the horizon is visibly tilted), help you focus on a particular part of a photo, and basic exposure adjustments. They're basic but perfect for this sort of application.
Finding People to Follow
There are a few ways to find new people to follow. The first way is by tapping the button that looks like a magnifying glass. At the top of the next screen, you'll see a search bar and you can search for friends there. Sometimes friends have, er, creative names so sometimes you'll have to get their names from them (if they don't use their real name). Once you start following people, you'll be able to see who they follow and who liked their photos and from there it's easy to find new people to follow (you just click the green follow button).