[Kaimetsu]

unprefixed variables are shared between all scripts. It is unwise to rely on them retaining their values between frames.
this. variables are specific to the script. Their values will be constant between frames, but you don't need that to be the case for simple iterators
temp. variables are specific to each execution of the script in which they are called, like local vars in C. They should be used if you need to use recursion, or if you think you might use vars with the same name concurrently.