Child themes: how to setup and use

Hi everyone,

About "Child Theme" setup ...

"but only enable the Child theme so only the child theme is available to use via Appearance > Themes"
... and ....
"The parent theme must be installed, but not activated, in your site"....

What's the diference between enable and actived?

About "Child Theme" use ...

"you can download a copy of the file you want to modify onto your computer, make the changes, and then upload it into your child theme folder".

Is this valid for any kind of file (CSS, PHP)...? And image?

Thanks!