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?"
"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?"
"Jruby support - unicorn s a better choice than thin but it doesn t support jruby"
from question "Thin vs Unicorn on Heroku"