Forest is a focus app made by Seekrtech, a small team in Taiwan. It launched in 2014.
When users want to focus, they plant a virtual seed and set a timer. The tree grows while the phone is left alone, and it dies if they leave the app to scroll. Over time the trees build a forest that shows how much focused time was spent. Friends can plant together, and a tree dies for everyone if one person gives in.
Forest works with Trees for the Future, and users can spend coins earned in the app to have real trees planted. It is a paid app on iPhone and free with ads on Android.