A "marker" in knitting is similar to a bookmark; it's a temporary placeholder that either reminds you to do something or shows you where something is supposed to happen. Markers in knitting can be used to mark areas for shaping, inserting stitch patterns, making buttonholes, or measuring.
Abbreviations
m - marker
pm - place marker
sm - slip marker
How to "pm" and "sm"
Most patterns that use markers will mean to use a marker that slips on to the needle.
When the pattern says pm or place marker, put the marker on your right needle, so the marker ends up between stitches.
Then, next time you encounter that marker on the next row or round, just slip it from the left to right needle.
Types of markers
Ring markers - metal, plastic or rubber rings that go over the needle. They should be large enough to move freely along the needle. This kind of marker does not get attached to the knitting. Once a ring marker is in place, you can't remove it from the needle until you get to it on the row.
Split ring markers - very similar to ring markers, but have an opening. These ones can be removed from the needle.
Removable markers - often plastic, these resemble small safety pins. They can be placed on the needle or on a stitch.
Needle and scrap yarn - you don't need to buy specialized gear for every technique. If you need to mark spots along a button band for buttons, for example, you can thread small pieces of scrap yarn into your knitting. Remove these pieces of scrap yarn later.
Do-it-yourself markers - I'm always losing my ring markers under the couch. In a pinch, you can make your own little markers with a piece of twine. Cut a length about 6 inches long, tie it firmly, trim the ends. Use this loop of twine as you would a ring marker.
Where to use markers
At the beginning of a round - Place a ring marker between the first and last stitches of a round.
When a pattern tells you - Many patterns instruct you to place markers. Even if you don't like to use markers, you'll want to follow the instructions and place markers because they will be used later in the instruction.
When you are working a chart or stitch pattern with a long repeat - if you are working a texture, lace or color pattern, markers are handy between pattern repeats.
Use a removable marker when you are counting - you often have to work x number of rows. Instead of counting from the beginning every time, place a marker when you count, and make a note on a notepad "marker at x rows".
When you're casting on - If you have a long cast on, it's difficult (and boring) to count accurately. I like to do this: cast on 20 (or 30 or 40) stitches, place a marker, and repeat. Then I count the sections instead of each stitch.
When joining seams - when joining a long or complex seam, mark off both pieces into quadrants with removable markers. This makes it easier to make a smooth join, because you line up the seam within each quadrant.