There is special term- memory leak. What it is? Looks similar to brain leak, or that old joke from old zombie movie: brains brains…!
In real world, the bug in memory management is described as memory leak. Every program use some memory (RAM, Video RAM, virtual RAM) to store some data. In modern operating system, program must request from OS for some memory. After using it, the program must release the memory. But there are bugs in software, and sometimes requested memory is a bit more than released. There is not a big problem, if the program is working for a short time- after the termination of process, all allocated memory is released (unless the OS have memory leak itself). But some software is running all the time computer is on. Especially in servers, where uptime is calculated in days, months and even years.
So, one buggy piece of software is Bittorrent client. Here is the result after running it for some days…
This image is more informative… 1.3Gbytes (!) of RAM is quite many for background file transfer.