Since you talk about AVR, you probably mean the format of the HEX-file? That is encoded as Intel HEX format.
EDIT Regarding your question from the comment:
I see the Header has the field "DATA". What exactly is in that field? The pure assembly?
Each line of the HEX file is called a "record". There are several types of records. Depending on the record type, the data contents have a different meaning. A "data record" holds the actual firmware/program data. That is lowest level machine code, not assembly. It represents exactly the data that resides in the flash memory after the device has been programmed.