When promoting your app, having a good one line pitch is crucial. You will need to be brief and have a great opening to get anyone to read more. Whether it's an email subject or tweet, make sure you can describe what your app does in one simple line.
You push a button and in five minutes a Mercedes picks you up and takes you where you want to go.
Example: Quit That! (one of my apps)
Easily track all the things you want to quit without being harassed by ads or limitations
Much of it is avoiding jargon and getting to the need the app is filling. Think of the opposite of how Microsoft may describe exchange server. :)
The Art of Writing One-Sentence Product Descriptions by David Bailey