Worried about overwriting data in PostgresSQL/Heroku set up

I have managed to get a database and hosting set up with Heroku.

My question is, when I am working on local target am I actually working on the live database?

My main concern is that I will lose data that may be submitted with a form on the site. So when I am working on local version am I working on a local copy or a remote copy? When I publish are only changes to the db structure deployed?

I am very confused.