A "memory leak" is just a bug that causes a program to lose memory. For example, here's some Perl code that leaks memory:

while (1) { 
   my $b; 
   my $a = \$b;
   $b = \$a; 
}
[download]

If you run that code it will eat all your system memory and then (if you're lucky) die. If you're unlucky your OS might decide to kill something else and cause bad things to happen.

Without seeing your code, or at least getting a good description of it, there's absolutely no way any of use can tell you if you have a memory leak. That could be what's killing your program, but I'd guess not - 300MB isn't very much memory really.

-sam