Markers are used during knitting to mark your place (as the name suggests). They can be used both horizontally and vertically.
When used horizontally, the markers are on the needle itself and passed along each time they are reached. This is really helpful for keeping track of complicated patterns, or for rows of hundreds of stitches to assist with counting. In this case, the markers can be closed (like a circle) or split (like these ones).
When used vertically, only split ring markers like these can be used. They're hooked onto the knitting at certain points to make counting rows easier, or to mark certain places on a garment (for example, marking at shoulder seams on a jumper).
These Clover split ring markers come in three colours - blue, pink and white - and are suitable for most needle sizes due to their flexible split.