difference between xsl:param and xsl:variable

I am considering using either <xsl:param> or <xsl:variable> when defining an <xsl:template name="myTemplate"> that will be called in another XSL file.

Legacy code seems to use xsl:variable when being used in <xsl:template match="legacyTemplate">.

So what's the difference between xsl:param and xsl:variable when using it in <xsl:template name="myTemplate">?