Compose, Alt, and Meta

[Bryan Scaringe]

Could someone explain the functions of the following keys
under a typical un*x setup:
        Meta
        Compose
        Menu
>From my understanding, Meta is just another "selection"
key, like Control or Alt.  That is, it's just there for
mapping special functions to it.  But is Meta the Un*x
version of Alt, or do they both typically exist together?

Thanks,
        Bryan Scaringe