Reference

umbraco.library
Function library for umbraco. Includes various helper-methods to save, load and transform data from umbraco. 

Especially usefull in XSLT where any of these methods can be accesed using the umbraco.library name-space. Example:
<xsl:value-of select="umbraco.library:NiceUrl(@id)"/>
Read the full documentation here...

umbraco template tags
- umbracoGetItem. Inserts the value of a given field
- umbracoMacro. Insert a macro
- umbracoTemplateLoadChild. Inserts the child template
- aspNetForm. Insert a server-side ran form tag for using runat="server" html-controls