To make short of the matter.
There are enough comments for the default xslt. We can also learn more from the parameter names. But first you need to learn the xslt grammar such as select value, traverse, custom template and so on.
The main process is as follows:
1. <xsl:template match="/">, need not to take much care because it is just a entrance.
2. <xsl:template name="dvt_1.body">, the showing of search result body part.
3. <xsl:template match="Result">, the most important part for your customization. You can customize your own structure and style here. Pay attention to this part.
4. <xsl:template name="ResultDetail">, it is also an important part since it includes the details of search result such as description, open in browser link and so on.
PS: The logic of the xslt is a little complex so you'd better keep the xslt as much as possible unless you believe you are not going to refer them.
Enjoy it.