Selenium, let's wait?

Automation is gaining popularity today. More and more young test automation engineers begin writing their first UI tests using Selenium. Unfortunately, modern web applications have become quite complex, and first steps in automation lead to unstable tests. A common error in this case - "test did not wait for anything." I will  explain wait mechanism and why "thread sleep" is a bad idea. I'll show mechanisms of wait from Selenium arsenal, how to write custom waiters and how to work with wait mechanism of non-standard objects.