XSL help

I am new to XSL:key.

Could any one tell for the following,
1) <xsl:variable name="result" select="key($index,$key)"/> what it does
2) To find long descripting, the existing xsl mapping for Type="PP" and
searching keyword for ZZ.  Question: without specifying the attibue name
such as Pkey and Type....How it searching the output of LongDes as ZO
Contracts ???