Newbie basic Q about how memory gets used?

Hi Folks,
Sorry to ask such a basic question.
I come from doing sites in Dreamweaver and never did anything with databases and WordPress. Now I’m trying to do a WordPress site and I am confused about how the php memory gets used. My host has given me 100MB as the limit and my question is NOT about changing that.

I just don’t get what “activities,” use up that memory???

I think I get that each plugin is a little program, and when it is active/running it takes memory. Like when I have lots of software programs open on my desktop computer.

Other than that I don’t understand how to avoid future problems because I don’t know anything else.

Q. If I upload lots of files into my wordpress theme/galleries, is it just the moment I am uploading them that uses that memory?

Q. Does having lots of photos on my site use my memory because it is sitting there?
Or does it use the memory only when someone comes to the site and goes through my gallery?

I would greatly appreciate any info guys – Thanks