Mainly because Luca asked me to, I'm going explain the field positions and a few other things about AFL football. Other people interested can use this as well.
Back Pocket - - - - - - - Full Back - - - - - - - Back Pocket
Half-Back Flank - - Centre-Half Back - - Half-Back Flank
Wing - - - - - - - - - - - - Centre - - - - - - - - - - - - Wing
Half-Forward Flank - Centre-Half Forward - Half-Forward Flank
Forward Pocket - - - Full Forward - - - Forward Pocket
Followers: Ruck - Ruck/Rover - Rover
Back Pocket: A Back Pocket is a defender who is, almost always, not the best defender. They usually man up on the 2nd or 3rd choice forwards or small forwards, the Forward Pockets. Due to modern tactics, the Back Pocket has become more strategic then back in the yesteryears. Back Pockets now are generally players who play more as a Half-Back Flank, which will be described later, and set up attacks out of the Backline, but do not move up the field like Half-Back Flanks.
e.g. Campbell Brown, Tom Harley, Dale Morris
Full Back: The Full Back is the lynch pin in a team's backline. The Full Back will usually take the opposition team's best forward, or a forward who is staying inside the 50 metre arc, usually the Full Forward. A Full Back is usually around the same height as a Full Forward, around 188-200 cm.
e.g. Matthew Scarlett, Brian Lake, Darren Glass
Half-Back Flank: A Half-Back Flank is a player who is not as defensive as a Back Pocket, but they still man up on an opposition player with the intent of negating them, usually the Half-Forward Flank. A Half-Back Flank will be more attacking then a Back Pocket as the will push into the midfield as their opposition does so too. The also can move up the ground towards the attacking 50 metres and do actually score goals. Back Pockets and Full Backs rarely do this.
e.g. Andrew McLeod, Heath Shaw, Tadgh Kennelly
Centre-Half Back: A Centre-Half Back is the player who takes the oppositions Centre-Half Forward. This position is not the same as a Full Back ,although they both are meant to stop the opposition forward which they are playing on from scoring goals. Generally Centre-Half Forwards will push up the ground into the midfield, so a Centre-Half Back is fitter and a better runner then a Full Back.
e.g. Trent Croad, Nathan Bock, Sam Fisher
Wing: This position is not hard to explain. It is basically like a wing in football/soccer. They run the sides of the oval and are generally fast and long kicks, although a trend which seems to be getting bigger is to have tall, athletic players on the wings so they can take marks over the smaller midfielders who would be playing on them, a la Matthew Richardson.
e.g. Chance Bateman, Adam Goodes, Matthew Richardson
Centre: As with most sports, they evolve, and this position as has evolved too. A Centre used to sit in the Centre Square and direct the ball flow moving through the middle of the ground. But in modern times, Centres have become fitter and now follow the ball around the ground like a Follower. Centres are typical midfielders who are fit, generally not that tall and have a good disposal of the ball(kick and handball).
e.g. Sam Mitchell, Jimmy Bartel, Scott Pendlebury
Half-Forward Flank: A Half-Forward Flank is a player who is a forward, but they push into the midfield and have more goal assists then traditional forwards. Generally they are short and fast players who can evade opposition players and give the ball to traditional forwards.
e.g. Steve Johnson(can play as a traditional forward too), Ryan O'Keefe, Brent Harvey
Centre-Half Forward: The Centre-Half Forward is, traditionally, the most important forward. These players push into the midfield and get the ball into their attacking 50 metres, but also are major contributors as goal kickers. A Centre-Half Forward is basically a more productive and useful version of a Full Forward. They are generally players who are big and tall, around 188-200 cm.
e.g. Jonathan Brown, Matthew Pavlich, Travis Cloke
Forward Pocket: Forward Pockets are the sideshows to the Full Forward. They can be tall forwards who aren't as good as the Full Forward, a medium sized forward who is awkward to match up on, or a small forward who gets the ball in general play and is not a key target in the forward line.
e.g. Cyril "Junior" Rioli, Paul Medhurst, Michael O'Loughlan
Full Forward: The main target in a team's attacking 50 metres, the Full Forward is a player who is usually the same size as a Centre-Half Forward, but they don't generally move up the ground the get the ball. They sit inside the forward 50 metres and wait until the ball gets to them. The are generally the best shot at goal, but in Hawthorn's case, the worst

.
e.g. Lance "Buddy" Franklin, Brendan Fevola, Matthew Lloyd
Ruck: A Ruck is a player who is generally the tallest player in the team. Around 200 cm is the average height. Ruckmen are the players who jump when the ball is bounced, and attempt to tap the ball to their teams advantage. Ruckmen can also be placed in the forward line to add some height. This is useful against undersized backlines.
e.g. Dean Cox, Aaron Sandilands, David Hille
Ruck/Rover: A ruck/Rover used to be a player who was taller then the average midfielder and would sometimes do the ruckwork in place of the Ruckmen. Alas, in modern times Ruckmen are fitter and faster so the Ruck/Rover has merely become another midfielder who follows the ball.
e.g. Adam Goodes, Chris Judd, Brad Sewell
Rover: Rovers traditionally are short midfielders who hang around the edge of a pack, waiting for the ball to spill out so they can get it. Nowadays, Rovers are just the same as most midfielders and follow the ball.
e.g. Sam Mitchell, Gary Ablett, Adam Cooney
Tagger: A Tagger is a player who sacrifices his own game to take out a very good player from the opposing team. Not all Taggers sacrifice their game though. Some are good enough to take an opposing player out of the game but still get plenty of possessions them self. A Tagger will take another player out by following the player wherever they go, so whenever the player they're tagging gets the ball that player gets tackled. Or the stick on the player so the ball won't go to that player.
e.g. Robert Shirley, Brad Sewell, Jared Crouch
Utility: A Utility is a player who is able to play in more then 1-2 positions. Utilities are usually tall and athletic, so the can play as a tall player or a small player. Utilites often switch between Foward, Back and Midfield.
e.g. Adam Goodes, Matthew Richardson, Lance Franklin
Key Position: A Key Position player is someone who holds down one of the 4 most crucial positions on the field. Full Back, Centre-Half Back, Centre-Half Forward, Full Forward. A Key Position player is tall so they can be formed to play in whichever position.
e.g. Jonathan Brown, Matthew Pavlich, Lance Franklin
Spine: The Spine of the field basically consists of the positions which go through the centre of the field. Full Back, Centre-Half Back, Centre, Centre-Half Forward, Full Forward, Ruckmen.
Set Shot: A Set Shot is when a player has either marked the ball or has received a free kick, and is having a shot at the goals.
There are still more things but they are too numerous.