What does a 13-bit program counter mean in PIC 16F84A and 16F628 microcontrollers?

Another question from me yet again...........................

I have been reading the datasheet of the PIC 16F84A, and PIC 16F628, and I found out that I did not understand what is meant by a 13 bit program counter....

What I mean to ask really is this: what is the meaning of the *BITs*, is it memory space or what, I just need an explanation of what it really means, pleaaaaaaassssssssssss I really need help on that, I have very little understanding of what it means

Was going to post, but Colin beat me to it.

Nice, thanks very much Collin, I really got some stuffs from what you said, really nice.... And thanks too Mike.. I know i would come over to ask more questions, and I would hope for answers.. I am highly grateful

Wow Colin, some answer.
Unfortunately several steps too far for me.
I thought (being very general about this) that the number of "bits" was a measure of processing capacity.
Do you need to know "the detail" to be able to set up basic instructions?
And how do you actually effect the zero-one changes?