The stem connects the steerer to the handlebars and for most road bike applications you choose a stem to tweak the fit of the bike.
Length & height
There are various rules of thumb for stem length and height, but there's no substitute for trying different stems to see what feels best for you, and seeking the advice of a bike shop that's experienced in fitting bikes to riders.
Don't assume that you necessarily want a similar set up to a pro rider who happens to be the same height and proportions as you. A sprinter like Alessandro Petacchi can get away with a 140mm stem because he's quite flexible, and because his main priority is to get very low on the bike in the last 300m of a race. For most of us, such a set-up won't be comfortable.
Most road stems have either a ten degree angle, which gives a slight rise or drop when they are fitted to the bike, or 17 degrees, which makes them flat in normal use. Time trialists will want to get the bar as low as possible without impairing breathing, while comfort and efficiency determines the rise for other types of road riding.
Leaving your fork steerer long, initially at least, will give you some leeway to play with stem height by moving spacers from the top of the stem to the bottom.
Speaking of spacers, there should always be a spacer on top of the stem if you have a carbon fiber fork, and the steerer should extend into the spacer. That way the full length of the stem clamp engages with the steerer, reducing the chance of damage to the top of it.
Materials & design
Most stems are either melt or cold forged aluminium and the more expensive stems go on to have a weight-saving and attractive engineered finish that is performed on the milling machine. Most stems are painted and a few anodized which is far tougher and scratch resistant than a painted finish.
The very few carbon fiber stems out there don't seem to offer much in the way of weight saving over the best aluminium stems, and many are actually heavier.
There area a few very, very light magnesium stems on the market. These have to be seen as for special purposes only: saving weight on very high-end road race bikes that are going to get frequently inspected by good mechanics. Magnesium needs careful handling and at least one importer that we're aware of has decided he doesn't want to be responsible for his customers not quite looking after these stems properly.
The front plate holds the handlebars in place and uses either four or two bolts to do this. Four-bolt front plates mean the manufacturer can get away with less metal and less closely monitored tolerances. There's nothing wrong with that but the lightest stems have well-made, closely toleranced 2-bolt front plates.
To reduce the risk of crash damage, the manufacturer's recommended torque settings for the stem bolts should be followed. These are sometimes printed on the stem or available from the manufacturer's website.
If there are no recommendations, or you don't have a torque wrench, the Allen key bolts should be carefully tightened evenly and just enough to prevent the handlebars from twisting in the stem when downward force is applied to the hoods.