Sidequests are just small parts that add to the main story line and may get you titles or better items.
Related ThreadsSidequest Information (SPOILERS) - last post by @ Aug 18, 2004
Pre-Future Sidequests - last post by @ May 23, 2012
Inn Requests and Sidequests - last post by @ May 7, 2012
Question about sidequests/costumes*SPOILERS* - last post by @ Aug 6, 2009
SideQuests??? - last post by @ Dec 10, 2008