You can share your app in an executable form (.apk) that can be installed on a device, or in source code form (.aia) that can be loaded into App Inventor and remixed.
Package the app (.apk file) by going to the "Build" menu on the App Inventor toolbar.
Select "Application (Save to my Computer)." A pop-up box should alert you that your download has begun.
Once it completes, you can email the app to your friends who can install it by opening the email from their phone. If you want to distribute it more widely, you can upload it to a website that both you and your friend can access. Note that people installing your app may need to change the settings of their phone to allow installation of non-market applications.
Choose File | Export Project to export the source code (blocks) for your project. The source code is downloaded in a .aia file. If you send it to a friend, they can open it with File | Import Project.