Diff between GI numbers and accession numbers - (Jul/07/2010 )

Rfam returns me accession numbers and NCBI seems to want GI numbers and I'm not quite sure what the difference between the two are?

Is there an easy script to go back and forth?

Spinach
(new to bioinformatic research)

http://www.ncbi.nlm.nih.gov/Sitemap/sequenceIDs.html