Postgres 9 : - How to interpret the EXPLAIN query results

Hi

Please help, how can I interpret the following results of my EXPLAIN query
?, I can see that the query will through 18586018 rows, but what’s the
meaning of cost and width?

GroupAggregate (cost=762381141.96..768375132.77 rows=18586018 width=32)

CONFIDENTIALITY NOTICE
The contents of and attachments to this e-mail are intended for the addressee only, and may contain the confidential
information of Argility (Proprietary) Limited and/or its subsidiaries. Any review, use or dissemination thereof by anyone
other than the intended addressee is prohibited.If you are not the intended addressee please notify the writer immediately
and destroy the e-mail. Argility (Proprietary) Limited and its subsidiaries distance themselves from and accept no liability
for unauthorised use of their e-mail facilities or e-mails sent other than strictly for business purposes.