Sure. https://vudu.me/child gives an explanation about child themes
Now, one thing I’m not sure of. The most recent version of WP (3.2) has a new default theme, twentyeleven. However it shipped with twentyeleven and twentyten
I’m not sure what the future plans are for twentyten now that it is no longer the ‘default’ theme
Also, with version 3.2, WP no longer overwrites every file everytime something is upgraded. Only the files that changed are swapped out.
However, even when using other themes in WP besides whatever the default is, it is best to consider a child theme. That way if the theme in use gets updated, your edits made in your child theme remian safe.
For a lot of theme work you may do, html and css knowledge are pretty much all you need. I don’t know php per se. I just have figured out enough to know what php is doing in the contect of WordPress. So once you can figure out what the php is doing, it’s the html and css that take care of the styling.