1st off, they're not silly questions...
1) "The garbage collection algorithm in Java is vendor implemented " Meaning if you run the EXACT same program at the EXACT same time on a Windows box, a Solaris box, and an HP-UX box with the -verboseGC (so you see when the garbage collector executes). All 3 will produce different results. The algorithm that decides how/when the garbage collector will run is different between different vendors.
2)what means of platform dependent? the exact implementation of is determined by the vendor (Sun, HP, Microsoft, etc.) and will be different between different platforms.
3)what are the realtime programming? Realtime programming is a program that is dependent on the timeliness and predictablility of the execution. (maybe every 3 seconds the stock ticker will be updated, if you click "sell" things will be sold THAT second, at THAT price. -- In Java because you can't truly control
thread scheduling -- you can't always guarantee things will happen in real time.