A topstitch is just a stitch simple straight stitch, which you can see on the outside of a garment. It is used primarily to hold layers of fabric together so that a garment sits correctly without warping of the fabric.
Because the topstitch is visible it takes a basic straight stitch and turns it into a thing of beauty or horror, depending on your stitch.
With the Janome Memory Craft 15000/12000/11000/10000/9900 all of the guess work has been taken out of how to achieve a great topstitch, because Janome have devoted an entire Sewing Applications section to Topstitching.
To get an amazing Topstitch:
Sewing Applications Menu
Select the Seaming menu
Your machine will now be set to the optimal settings for the perfect topstitch