Stems are for connecting the handlebars to the head tube, as the geometry of road bikes make it unnatural to mount the bars directly on top of the tune in the way the mountain bikes or hybrids have their bars mounted.
The stem you choose needs to ensure that your reach is ideal for the size of the bike. Whilst the height of the bars can be slightly adjusted with spacers, the stem's length is fixed and if it is wrong it will need to be changed to allow you to ride comfortably and without injury.
Stems come in a variety of styles from brands such as Deda, who are experts in cycling components like handlebars and headsets. We also have stems for sale from Trigon and stems in a range of colours so you can match the stem to the colour scheme of the bike.