A leak is a (usually) unauthorized disclosure of inside information about government activities. There are, of course, trial balloon leaks that are authorized, but the leakers have to remain anonymous.
On the other hand, dumping is authorized release of (usually) embarrassing information. It is usually done late on Friday to minimize news coverage, and often in such large quantities of documents that reporters can't possibly review it all before writing their stories. Nothing new here. But Politico today has a neat article documenting how many items embarrassing to the Obama administration have been dumped on Friday. The practice goes back many years because it serves the political interests of embarrassed administrations.