edit: This answer is wrong. I'm leaving it up as an example of how easy it is to be mistaken about something you think you know very well. Thank you to everyone who pointed out my mistake.
A memory leak is: A programming error. Your software borrows some memory from the system, uses it, and then fails to return it to the system when it has finished. This means that that particular chunk of memory can never be used by any other programs until the system is rebooted. Many such leaks could use up all of the available memory, resulting in a completely useless system.
To prevent memory leaks, practice RIIA, and always test your software. There are plenty of tools available for this task.
