How does particle Start Speed and Velocity over Lifetime interact? How about Start Size + Size over Lifetime?

I'm a little unclear on what the relationship is between the particle "start" attributes and the "over time" attributes. For example, it seems like the "over time" attributes have more influence, but if I set the start size to 0, particles stay at scale 0. Is "over time" a percent then? If so, a percent of what? Similarly, for velocity, does the "over time" velocity get added to the start speed? Or is it multiplied? Or what?

Sorry if this is a basic question, but it's something I've never been clear on.

Thanks!