If you are troubleshooting slow MySQL queries and are confused with the tabular output of the EXPLAIN command, you may wish to check out my little MySQL EXPLAINer service. You upload a CSV file with whatever output
EXPLAIN SELECT has given you, and the query execution plan are is presented in form of a tree.
The CGI script is actually just a web front-end to the nice command-line tool mk-visual-explain, which does all the hard work of parsing EXPLAIN output. All due credit goes to the developers of Maatkit - read their documentation and slides referenced from their site if you are really interested in the technical details.