Link juice is the currency of Google. In other words, it is the secret sauce that allows you to beat your competitors for rankings in Google. Sure, there are some other factors that Google uses to calculate rankings (see Search Engine Optimization), but link juice is by far the most important aspect of dominating Google and in order to understand Google, you have to understand link juice.
Link juice is a term used for the benefit and pass on the link, and not all links are equal. Was first coined this term by the network marketing consultant, Greg Boser. Google version of link juice, is almost the same location and have Bing points, but it is not known what the relevance of factors juice in this result.