There are various versions of spanning tree protocol, including IEEE, DEC and IBM variants. The one you are most likely to come across these days is IEEE's 802.1D 1998 version, but that has changed over the years. The current standard is at
http://standards.ieee.org/getieee802/download/802.1D-1998.pdf
As mudassir states, this is augmented by other IEEE 802.1 standards, including 802.1w Rapid Spanning Tree available at
http://standards.ieee.org/getieee802/download/802.1w-2001.pdf
and 802.1s Multiple Spanning Tree (not available for free download as far as I can tell - let me know if you know otherwise!).
Hope this helps
