Optimistic UIs in under 1000 words

Igor Mandrigin gives an illustrated introduction to optimistic UIs. You may have not heard of optimistic user interfaces yet, but they're all over the web.
Optimistic UIs don’t wait for an operation to finish to update to the final state. They immediately switch to the final state, showing fake data for the time while the real operation is still in-progress.
Optimistic user interfaces are a core concept in today's web applications. Our interfaces expect a user action to succeed, so they don't need to wait for a server response to move on to the next task.
It is a very simple concept behind a fancy name. Yet it can make a huge impact on your users’ happiness. Firstly, it makes your app feel faster. User can start doing something else while your app uploads a Hilarious Betty the Cat Picture or posts a Smart And Ironic Comment to a discussion. Secondly, it streamlines the experience by removing unnecessary states and distractions. The app will look simpler and more friendly.