What is the Codex Sinaiticus?

The Codex Sinaiticus contains what is considered to be the oldest existing copy of the New Testament. But it's not the same as the current version of the New Testament. Literally thousands of changes were made. Who made these changes and why? 150 years after Constantin von Tischendorf discovered the Codex Sinaiticus at Mount Sinai this ancient document is still relatively unknown and unappreciated, it's importance deliberately marginalised.