whats the difference and similarities of joint p.d.f and p.d.f?

Answer 1

A joint pdf is what you have when you've got two (or more, but two is the most common use in a probability/stats class) random variables, say x and y, and they behave in some fashion together... for instance

fxy(x,y) = e^x * y^2

This is very much like a pdf for just one random variable, say x, with function

fx(x) = e^x