How to add target list in campaign?

I am new to zoho and my aim is to use it mainly for tracking marketing campaigns. I own an estate agency and we do have quite a volume of marketing going out every month. I understand the meaning of leads in zoho but my question is this. When I do a targeted campaign with letters to hundreds of people do I add them in zoho as leads or do I keep them in the csv file and add the file in the campaign? If yes then how can I add a file to a campaign?

Any insight would be helpful as I am really new to this.

Thanks