Kelly,
If the lead isn't in your SFDC system, the email will be sent to whom ever the flow step indicates (normally specified in the "other emails" box). If the lead is in your system a contact, it will send to the Contact's owner if you specify in the flow step that you want it sent to the Lead Owner.
The Lead Owner field in Marketo is actually a 2-to-1 mapping to the Lead and Contact owner fields in SFDC, so specifying Lead Owner in the Marketo flow step will automatically send to the correct owner (if one was specified in the Lead/Contact record).
Let me know if you have anymore questions!
Jesse