What is stored in a transaction log file?
The transaction log stores every transaction and the changes made by those transactions to the database. This answer gets very complicated when going into detail but generally it stores the following. 


Modification Operations
Log sequence numbers (LSN)
Row data. Before/After/Partial/Meta-data, depending on the operation. 
Reserved space for rollbacks


It does not store queries or t-sql statements. Please refer to this overview by Microsoft. 

https://msdn.microsoft.com/en-gb/library/ms190925.aspx

The specific format is not fully documented and is subject to change betwen versions.