The main unit of work or action in your application - usually a screen. Technically, an Activity is a class which extends the Activity base class so it implements functionality defined there, such as the lifecycle methods onCreate(), onStart() etc

The Android Developer Challenge was a competition for the most innovative application for Android. Google offered prizes totaling 10 million US dollars, distributed between two phases of the competition. The first phase accepted submissions from 2 January to 14 April 2008. The 50 most promising entries, announced on 12 May 2008, each received a $25,000 award to fund further development. The second phase ended in early September with the announcement of ten teams that received $275,000 each, and ten teams that received $100,000 each.

Android is a software platform and operating system for mobile devices, based on the Linux operating system, developed by Google and later the Open Handset Alliance. It allows developers to write managed code in a Java-like language that utilizes Google-developed Java libraries, but does not support programs developed in native code.

This is the extension given to Android installable files, for example foo.apk. Android will install the application from this. Note that for distribution the application must have been signed. Developers can create an unsigned apk file, so it it not clear from looking at the filename alone if the program can be installed.

'Apps' are an abbreviation of the word 'applications',