Journalling is used when some blocks on the disk are scheduled to be overwritten. Instead, the blocks are written in a separate location (the so-called journal). At a later point in time, the data blocks get overwritten. Once the data is safely on the disk (via Forced Unit Access (FUA) or a flush track cache-type operation) we can then truncate the journal.
Log-structured systems do not do this. Instead, the achieve crash safety via never overwriting data; they always write elsewhere (into something called 'the log'). Eventually, too much of this log is filled with garbage, stale data. This is when 'cleaning' occurs. However, cleaning is expensive and slow. You must read a large amount of data, figure out what is alive/dead, and then write it all out.
F2FS's contribution to FS design seems to be handling this cleaning operation in a more graceful manner.