Vw stands for viewport width — a percentage of the full width of the browser window. 1vw resolves to 1% of the current viewport width. For example, if the viewport is 50cm wide, 1vw=1/2cm.

The difference between % and vw is most similar to the difference between em and rem. A % length is relative to the containing element width, while a vw length is relative to the full width of the browser window.

It's become very popular to use viewport units for responsive typography – establishing font sizes that grow and shrink depending on the current viewport size.