The short if it is that the typical URL
http://localhost:10039/wps/portal/!ut/p/b0/04_Sj9CPykssy0xPLMnMz0vMAfGjzOKd3R09TMx9DAwsTC0MDTwdPULNzYJdjAxczfXD9aPwKnE0hiowwAEcDfT9fPLT01NT_EtL9Auys9McHRUVAdjo3OY!is an XML representation of the portal page compressed and encoded with a proprietary algorithm. It turns out there are several alternate URL schemes for WPS, but this one is the most common variation that everyone is familiar with.
No need for me to go into details - the document explains it quite well.
EDIT 09/30/2013: Here's a URL that describes REST services which will encode/decode portal URLs: