Making the best use out of Mainenance Plans on SQL Server

hello in the maintenance plan link:

http://www.codeproject.com/Articles/105573/Making-the-best-use-out-of-Mainenance-Plans-on-SQL

I want to know whether to take log backup, backup should be selected across one or more files

means that expires after xdias

I created the two backups: bak (full) and differential (trn), but the log can not be restored