(In the interest of full disclosure, I was a defenseman myself once, back in the day.)
So, as a refresher from last week, the basic terminology for a hockey rink is this: between the blue lines at center ice is the neutral zone. From the blue line to your goalie is the defensive zone. From the blue line to the opposing goalie is the offensive zone.
A defenseman's home is from in front of his own goalie to the opposite blue line-ish. Some defensemen will cheat in closer when the forwards are buzzing around the net, but at least one should stay back. To cover for for breakaways, if nothing else.
They also skate backwards. Some forwards can do that as well, but not many are comfortable doing it. Goalies skate backward as well, but they don't have to for any significant distance. Skating backwards, once you get the knack of it, is actually easier than skating forward. It takes a lot less energy, and it's also faster than skating forwards is, which is how the defense stays in front of opposing forwards so easily.
A defenseman - or the defensive tandem; also just called "the defense" - is roughly the same as the forwards in basketball, particularly in their own end and under their own basket. In football, defensemen roughly correlate to cornerback or safety and outside linebacker. Sometimes you have two of one, or two of the other. If at all possible, you want one of each in a tandem.
For defensemen, as they work in pairs, they often are just right defense and left defense. They pretty much stick to their side of the ice, and that's about as far as it goes. So far as it goes, the job of a defenseman is pretty simple; stop the forwards from the other team from scoring, and pass the puck up the ice to the forwards.
Ideally, however, you want one that's a stay-at-home defenseman - that is, they play pretty conservatively, and don't take a lot of shots or get a lot of goals. Then you want the other to be more of an offensive defenseman. And offensive defenseman is one who tends to skate a lot, makes plays, takes some shots. A particularly offensive player will get 15-20 goals a season, but they'll rack up three times as many points as assists.
The defenseman's shot is typically the slap shot. (As shown by the incomparable Nicklas Lidstrom from the Detroit Red Wings in the picture above.) The slap shot can get a lot of speed behind it. The NHL's Skills Competition, during their All-Star Weekend festivities, includes the hardest shot challenge. The reigning hardest shot winner in the NHL is defenseman Zdeno Chara from the Boston Bruins, with a slap shot clocked at 108.8 mph (174 kph).
And, yes, goaltenders still throw themselves in front of those things to stop goals from being scored. And they have the bruises to show for it, too. But more on them next week.
Despite being what seems like a simple position, a defenseman doesn't really hit his prime until his late 20s. There are a lot of nuances to learn, and that can take a while. You're playing between the forwards and the goalie, so you have to figure out how you fit into the grand scheme of things - more so than any other position.
Goalies can be very particular about how they want the defense to play in front of them. Some want shots to hit them, and others want shots blocked, among other things. And forwards can either be very helpful and play as defensively as the defense, or just wait for the first pass out of the zone and let the defense do their thing. A young defenseman usually make quite a number of mistakes before he finally figures out how things work.
One of the interesting aspects of the position is the fact that it's essentially invisible to fans. If a team wins a game, it's because the forwards were awesome and scored a lot of goals. If a team loses a game, then it's because the goalie sucked and he shouldn't have let in so many goals. The defense is rarely praised nor damned for the outcome of most games.
But the fact of the matter is that a good defense can make a decent goalie look great, and a bad defense can make a great goalie look very average. They can set up the forwards for fantastic goal-scoring opportunities, and they can turn the puck over repeatedly, setting up fantastic goal-scoring opportunities for the other team. They are in a supporting role, I guess, but one that does have significant influence in a game.
But, you have to be paying attention, or else you'll get caught up in the impossible shot that just went in past the other goalie or the insane glove-save that your goalie just made.