From BOL :
A log backup taken from a possibly damaged database to capture log records that have not yet been backed up. A tail-log backup is taken after a failure, in order to prevent work loss and can contain either pure log or bulk log records.
Code from BOL to take a tail log backup
BACKUP LOG MyAdvWorks_FullRM
TO MyAdvWorks_FullRM_log1
WITH NO_TRUNCATE
GO