You can see the full list of libraries on your system using the iseries navigator. If you just want the IFS, use WRKLNK.
QDLS is one of the file systems in the OS/400 Integrated File System, as is QSYS.LIB. The AS/400 folders you are familiar with are in this file system. The WRKFLR and WRKDOC commands, among others, are used to manage these folders and their content.