Second one's a trojan. First one's a worm; it doesn't actually infect files or disks the way viruses did "back in the day". It relies on tricking the user into executing a program when the user is not expecting to be executing a program.
Compare to Windows worms that run automatically, or viruses for Microsoft platforms that infect files that are commonly shared.
I repeat my statement that you don't need anti-virus on the Mac. You simply need to have common sense.
I don't think it's too naive of a viewpoint. There are legitimate security concerns on the Mac, just as there are on other operating systems. My point was that you don't need cpu-cycle-sucking memory-resident antivirus programs the way you do on Windows.
