It sounds like you've got a huge transaction that has remained open which has done a lot of work. Run DBCC OPENTRAN on the database and see how long the oldest transaction has been open for. You'll probably need to kill that transaction (or have the user commit if it's someone in Management Studio). Then the log will clear automatically.
I could not put this as comment so here it is for you. It is not compete answer to your question because your question can either be correctly answered by person working in Microsoft or Paul Randal(I guess). All I can say for all paractical purposes every information is logged.
You can read the contents of log file using undocumented command
select * from fn_dblog(Null,Null)
If you run it you can see lot of information related to database, pages,extents,locks ect. But it would be difficult for you to extract information from it as it requires a level of expertise to decipher the output.
If you read Books Online document SQL Server Transaction Log architecture and Management it says
Many types of operations are recorded in the transaction log. These
operations include:
•The start and end of each transaction.
•Every data modification (insert, update, or delete). This includes
changes by system stored procedures or data definition language (DDL)
statements to any table, including system tables.
•Every extent and page allocation or deallocation.
•Creating or dropping a table or index.
Log records for data modifications record either the logical operation performed or they record the before and after images of the modified data. The before image is a copy of the data before the operation is performed; the after image is a copy of the data after the operation has been performed.
AFAIK there is no information about query which was fired but the changes which query made is written in transaction log. Changes made to page, extents , locks that were taken, resources that were locked.
Best Answer
According to the SQL Server Transaction Log Architecture and Management Guide, data pages are modified in cache first (logical write). This occurs the same way regardless of the database recovery model, with only the level of logging detail being different. A transaction log record containing the modification is created and written to the log buffer cache. These log buffers are always physically written to disk before the modified pages (write-ahead logging).
Data pages may be written to disk by lazy writing, eager writing, and the checkpoint process. The log buffer is flushed to disk before the corresponding data pages are written to the data files by these processes.
Transaction log buffers are physically written to disk when 1) they become full, 2) during COMMIT when the log buffer contains data for a durable transaction, 3) a checkpoint, or 4) a modified page is written to disk by one of the aforementioned processes.