It is a 150 billion page web archive of web pages as they were before they changed. Think of it as a perpetual inventory of web pages, showing what each page looked like at any one point in time. It has documents, videos, audio, and many interesting things to pleasantly waste your time for days.
It serves about 500 queries per second from the approximately 4.5 Petabytes (4.5 million gigabytes) of archived web data. The cluster of computers and the Modular Datacenter acts as a single massive computer.
The Wayback Machine is a tribute to the Rocky and Bullwinkle Show's "Waybac Machine" which in turn was a reference to the Univac Sherman and Peabody live on.