Just one form of finance available to a company to fund the use of Capital Equipment. Containing many tax advantages, and elements of a 'residual value' of the asset making this form of finance quite cheap. The asset is held as security against this obligation to pay a rental.
A funny finance term - but really very simple. Similar to hire purchase - to buy a car, but for companies. You don't own the car until to have made all the payments, but you have use of it in the meantime. Leasing is nothing more than 'company HP' with a few tax dodges, and different flavours, that make it cheap.