Um, Tiger is an Operating System. iTunes is an Application. Two very different things (if you know anything about computers).
n00b
An operating system is just an application that runs straight off the hardware. Other applications run inside it. Not that much different.
In fact you can have an application running inside an operating system inside an application inside an operating system inside another operating system inside the BIOS inside the motherboard. My example:
Appleworks > Mac OS 9 > SheepShaver > Win 2k > DOS > BIOS > Motherboard.