How do I use xsl:apply-templates

As a non-programmer, I am trying to learn XSLT so I can transform XML docs into Adobe InDesign or Quark compatible XML. I pretty much got the basics, but I'm still unclear on xsl:apply-templates. If you just match the root node, then the apply-templates command traverses the entire source tree. But if it finds a template that, say, matches a child node that you created, you may or may not have to add an additional xsl:apply-templates in order for it to continue processing child nodes. So what is the rhyme or reason to that? If I have templates do I always need to add an xsl:apply-templates command in each template?

Thanks so much for your advice.