You can merge any piece of data held against the contact into an email (or the email's subject line). When editing an email you can use the drop down mergefield selector to automatically insert the mergefield with the correct syntax into the email. Alternatively you can type in merge.
The general syntax of the merge looks like this:
Where 'fieldName' is the name of the data field that you want to merge in
and 'value' is the text that should be used should data not be available
If you wanted to merge in the contact's name you might construct a merge like this:
Notice the default option. The value entered as a default will be used if, for a particular contact, there is no data on the record to merge in. That means that specifying a default value will avoid blank merges.
If you add custom data fields onto the contact record these will automatically become available to merge into your email. Because of this it is not possible to include a definitive list of possible mergefields in this documentation.