A little over two years ago a website started up to help users collate and share images on the web. Within a year its users were growing from the thousands to the millions and its popularity was spreading from the US to Europe. Suddenly people were sitting up and taking notice of the Silicon Valley startup. Now, according to comScore, in February 2012 Pinterest hit 17.8 million monthly users in the US, up over 6 million users from the January figure of 11.7 million, making its rise amongst the fastest of any site to ever hit the web. Yet a lot of people still only have a vague idea what it’s about.