Difference between Thread.Sleep() and Selenium.setSpeed()?

selenium.setSpeed
 
1.Syntax: selenium.setSpeed(string time in milliseconds);
 
ex: selenium.setSpeed("2000") - will wait for 2 seconds
 
2. Runs each command in after setSpeed delay by the 

number of milliseconds mentioned in setSpeed.
 
thread.sleep
 
1. Syntax:  thread.sleep(integer time in milliseconds);
 
ex: thread.sleep(2000) - will wait for 2 seconds
 
2. Waits for only once at the command given at sleep.