"generateReport" will generate the requested report (for a specific date range, website and in the requested language).
"sendEmailReport" will send the report by email to the recipients specified for this report.
You can also get the list of all existing reports via "getReports", create new reports via "addReport",
or manage existing reports with "updateReport" and "deleteReport".
See also the documentation about Scheduled Email reports in Piwik.