FMQ is free motion quilting (where you drop the feed dogs and use the quilting/darning foot) SID is stitch-in-the-ditch, where you sew in the seam line of each row. SID is best done using a walking foot to avoid the layers shifting.
When doing FMQ you generally start in the center and work your way out.
When doing SID you generally start in the middle of the quilt but at the edge of a seam.
Hope that helps.