Is leaked memory freed up when the program exits?

The OS executing your program usually does cleanup memory that is not freed explicitly and handles that are not closed explicitly, but this is not guaranteed by the C++ standard. You may find some embedded device that do not free up your memory leaks.

That being said Windows and all distros of Linux that I've ever seen do free up memory leaks.

You can easily create a huge loop of memory leaks though to test it out yourself. Watch your RAM usage grow and then close your program. You will see that the RAM usage goes back down.

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Another consideration when using C++ is that if you aren't deleting your heap allocated memory then your destructors are also not being called. Sometimes you will have other side effects as well if your destructors aren't called.

Yes, a "memory leak" is simply memory that a process no longer has a reference to, and thus can no longer free. The OS still keeps track of all the memory allocated to a process, and will free it when that process terminates.

In the vast majority of cases the OS will free the memory - as is the case with normal "flavors" of Windows, Linux, Solaris, etc. However it is important to note that in specialized environments such as various Real-Time Operating Systems the memory may not be freed when the program is terminated.