An aneurysm is a balloon-like swelling of a blood vessel due to a weakening of the wall of the vessel. It's only a problem if it bursts, when, depending on exactly where it is, it may be fatal pretty much immediately. But it can sit for days, weeks or even years, and you may not even know it's there. If the doctors spot it, they can take steps to repair it before it bursts, and you escape the worst effects.
An embolism is kind of the opposite. It's a blockage of a blood vessel, and can lead to death of the tissue that's normally fed by the blocked vessel. That may be crippling to some degree or fatal, depending on what tissue we're talking about (obviously, if it's the brain or heart, that's not good). But the blockage is not there one minute and it is there the next. There's no grace period. If you have an embolism, you are immediately fucked, to whatever degree is your destiny.
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