The ITIL Triangle
This is a diagram that describes the relationship between ITIL, the ISO20000 service management standard, and your own in-house procedures.
Essentially, this top down illustration starts at the highest level with ISO20000, the BSI standard specification for IT service management. The next level presents a code of practise for ITSM (eg: PD0005, which was produced by CCTA, ITSMF and DISC/BSI). ITIL itself is the third layer, with in-house procedures represented by the bottom layer.
The ITIL Triangle diagram itself can be viewed at ITIL and ITSM World, which offers a toolkit including most of the key documents.