Memory that is not deallocated when it is no longer needed, and is no longer "reachable". For instance, in unmanaged code, if I use "new" to instantiate an object, but I don't use "delete" when I'm done with it (and my pointer has gone out of scope or something).
The best way to prevent them probably depends on who you ask and what language you are using. Garbage collection is a good solution for it, of course, but there may be some overhead associated with this, which isn't a big deal unless you performance is your primary concern. Garbage collection may not always be available, again, depending on the language you are using.
Alternatively, you can make sure you have the appropriate deletes and/or destructors in place. There's a lot of methods and tools to detect memory leaks as well, but this will depend on the language and/or IDE you are using.
