The Colossus was the world's first large-scale, electronic programmable computer, preceding the ENIAC - which most computer historians had credited with being the first electronic computer - by almost two years. The existence of Colossus was not generally known until 1975 when a picture of it was declassified; in 1983, how it functioned was finally described; and in 1996, the US government - not the UK - declassified exactly what Colossus was used for.
The BBC story has a video interview with Captain Jerry Roberts, the sole surviving member of Testery, a code breaking unit that used Colossus to break wartime codes beginning on the 5th of February 1944. Colossus required 1500 vacuum tubes or "valves" and could process 5,000 characters per minute. A later model doubled that speed.
Professor B. Jack Copeland wrote an article for the 2004 IEEE Annals of the History of Computing called "Colossus: Its Origins and Originators" as well as edited a book called Colossus: The First Electronic Computer which was published in 2006 for those wishing more information.
A total of ten Colossi were eventually built.