Thin is an EventMachine-based web server written in Ruby for running Rack-based web apps.
A unicorn is a magical weaponized HTTP server for Ruby.
"I m running unicorn which is about 40 faster than thin on celadon cedar"
from question "Rails Performance Tuning for Production?"
"Jruby support - unicorn s a better choice than thin but it doesn t support jruby"
from question "Thin vs Unicorn on Heroku"
"In production it is much better to use a more sophisticated server like phusion passenger or unicorn since they have better performance than thin mongrel or webrick"
from question "How do you choose your HTTP server in Sinatra?"