The following resources will answer your question for General Email Limits and Mass Email Limits.
Single Email limit and mass email limit Of a enterprise Org
500 for mass.
Single Email Limit and mass email limit Of a Professional Org
250 for mass.
Single Email Limit and mass email limit Of a Unlimited Org
1000 for mass.
Emails sent via Salesforce will contribute to the daily mass limit except those sent to internal e-mails, including partner users. The General Email limits include things like attachment sizes, for example, which have their own limits.
With regard to Single Email Limits, all I can find is that
You can send 100 emails per SingleEmailMessage.
I imagine this is the same across all orgs as this resource doesn't mention anything except that limit for all editions. I could be wrong on this though.
This resource should help you as to the difference between Single and Mass E-mails, as explained there:
Single email in UI can be used by workflows, alerts and notifications etc, where as mass emails can be used in the UI to send mass emails to contacts and leads using a template.