Tonight when you go to sleep, you have a second longer at sleep than you did last night. Sorry, its not much but it does have wide ranging consequences to those machines that rely on time being so accurate. Computers that require the time to be accurate will have to be updated, these include financial institutions such as the stock markets. Most institutions will be ready for it given that they change times twice a year, once about end of October and the other end of March.
The reason for the leap second is because of the time it takes the Earth to rotate round the sun is not always precisely 86,400 seconds a day, it can be affected by the pull of the Moon, the Sun and even the molten iron core. To ensure that time is correct, the leap second was created in 1972.
The Metro newspaper of the United Kingdom came up with second facts that are interesting to read. For example, during a second of time, 4.3 people will be born but only 1.8 there by increasing the number of people on this Earth. 11 trees will be felled/chopped down in the Amazon rain forest and 1,120 tons of carbon dioxide will be released. Depressing reading when you think of it.