If this is so elementary as to detract from the discussion, please ignore it.
> What causes a mutation? Is it a flaw, a malfunction, randomness, or what?
In short... yes. Mutation can be caused by radiation bustign up bits of DNA, a simple error during replication (leaving DNA out, replicating a strand over and over, copying incorrectly), a retro virus inserting its DNA into the host cells DNA, etc.
> Is survival of the fittest a key part of ToE?
Obviously. If mutations made no difference, and an improcved animal had no better chances than the original, then evolution would not occur as we've known it.
> If so, what does fittest mean? Does that mean the survivors are the best overall of a particular species or organism, or just the ones that happened to have survived the particular occurring phenomena?
Largely the latter. Let's simplify: let's say humanity was reduced to two small town, one in Iowa and one in Japan. Let's say it was due to a global nuclear/biological war. Both groups have been ravaged by disease and radiation; mutations are popping up everywhere. Over then next ten generations, the town in Iowa becomes populated by genetic supermen... they are immune to disease, they can pick up boulders bare-handed, they are smart as tacks and can eat dirt and breath chlorine if they need to. But the Japanese town has been reduced to sickly, stunted and rather dimwitted troglodytes.
And then the Yellowstone supervolcano explodes in Wyoming, covering the Iowa town with three meters of red-hot ash. All the Iowan supermen die. The Japanese trogs survive by huddling in caves unto the weather clears, and then go on to re-populate the planet.
Well, in this case, the trogs win out on "fittest." "Fittest" is not one-size-fits-all concet, but is defined as those characteristics that will lead the the best chances of reproduction in the existing environment. If the environment changes non-uniformly, then a species that would otherwise be seen as weak and doomed could easily become dominant.
That's where the "random chance" comes into play on a big scale.