The edge of your knitting is very important, especially if it's going to be the finished edge of your work. When the edge of your work is going to be exposed using a slip stitch at the edge is the perfect way to create a smooth, attractive, and even edge stitch. There are a couple of ways to do this in your knitting that create different effects.
There are also some times where it's not a good idea to work a slip stitch at the edge of your work. This would be where you need to pick up stitches along the edge after you are finished. Usually you pick up in a ratio of 2 sts per 3 rows or 3 sts per 4 rows. When you have a slip stitch edge it creates one 'slip stitch' for every 2 rows so you won't have enough edge stitches to work with.
Creating a Chain Edge in Garter stitch
The first way to so this is to slip the first stitch of every row purlwise with the yarn in the front. Then when you want to work the next stitch you need to move the yarn to the back.
You can create the very same edge by slipping the last stitch of the row. When you reach the last stitch, you bring the yarn to the front, then slip the stitch purlwise.
When you turn your work the yarn is at the back where you need it and you just knit again to the last stitch.
This is what your chain edge on the garter stitch will look like when you're finished.
Creating a Knotted Edge in Garter stitch
To create an even 'knotted' edge to you knitting on the first stitch you keep the yarn at the back and then slip the stitch knitwise. Then you just knit to the end of the row. This creates a tighter edge on your knitting.
Here you can see the 2 different types of edging side-by-side. The bottom one is the chain edge and the top one is the knotted edge.