Tonight, at 23:59:59 on June 30, a leap second will be added to Coordinated Universal Time (UTC). Instead of the clock rolling around to 00:00:00 on July 1 as per usual, the time will actually jump to 23:59:60 before finally ending up in July a second later. The previous leap second, which was inserted in June 2012, caused a number of high-profile technological issues, and it's likely that the leap tonight will cause some problems as well.
Leap seconds are necessary because of the difference between civil time (i.e., the everyday time systems that pesky humans use) and the actual mean solar day (i.e. how fast the Earth rotates). In UTC, which is what other time zones are based on, a day is defined as 86400 SI seconds. How long the Earth actually takes to rotate on its axis is somewhat varied, however: for the past few centuries, the rotation of the Earth has been slowing, causing mean solar days to get ever so slightly longer.
In an attempt to ameliorate the artistic differences between UTC and Earth, the leap second system was introduced in 1972. Leap seconds can be inserted (or removed) at the end of June or December. There is no regularity in the system, though—nine seconds were inserted between 1972 and 1979, but only three seconds have been added since 2005—which is problematic in domains that demand regularity.
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