Most stitch markers are the metal ring and are used as Susan described.
If you have made a decrease or increase along a row, you pop a marker on the right needle and it acts as a marker or signpostso that you can tell whereabouts you are, should you lose track on a complex row.
I'd not been knitting very long, in fact, had just finished a baby's cardigan, when a well-intentioned but overly-optimistic friend thought I could tackle lace.
I chose the basic Feather and Fan pattern and got into trouble you don't want to know about.
I decided it would be more use if I could count out and mark the continual yarn overs and decreases first
- that way, all I had to do when the counters were on the needle was concentrate on the knitting and tension.
I started a range of markers made like this
and now I sell the solid rings as well to be used like regular markers
Anyway, hope that explains things a bit.