People often cry "memory leak" when a program is using lots of RAM, but it's a bit more complicated than that. Linus at Techquickie breaks down the truth behind memory leaks in this video.
Memory leaks are a very specific kind of high RAM usage. Usually, when a program stops a certain task, it will keep some stuff in memory in case you re-launch that task later (which is good!). However, it will mark it OK to "clear" in case your system needs that RAM for something important. Memory leaks happen when a program doesn't mark that finished task as OK to clear -- meaning it just stays in memory forever until you close the program completely or restart your computer (which is bad).
Unfortunately, memory leaks are usually the result of bad programming, which means there's not much you can do to fix and prevent them. You just have to restart occasionally or, better yet, stop using that program. Check out the video above for Linus' in-depth explanation.
Memory Leakage as Fast as Possible [Techquickie]