An Android activity presents a visual user interface for a focused task. It contains a hierarchy of views that provide interaction and visual content within its window. Activities are declared in the Android manifest file and can launch other activities using intents. An activity transitions through various states as it moves between the foreground and background. The activity lifecycle callbacks notify an activity of state changes like create, start, resume and pause. Activities are managed in a stack, so starting a new activity places it on top of any existing activities.