When you use campaigns to send emails via the connector, marketing cloud imports the campaigns as a Data Extension (and is visible in Salesforce Data Extensions for days).
What this means is that for fields that are in the campaign you can reference them with personalization strings as you would do when you were sending via
For example, if you have a
First Name, you could access that field by entering
Dear %%First Name%%
To access difference objects (e.g. Contact / Lead) you would have to use the RetrieveSalesforceObject function.
%%[ var @rs
Set @rs= RetrieveSalesforceObjects('Lead', 'Id, FirstName,LastName', 'region', '=', 'west')
This will usuall return a rowset from which you would have to use the Field and Row function to narrow in on your fields.