Teams
A team consists of seven players:
3 Chasers
2 Beaters
1 Seeker
1 Keeper

The Field
Prime seats would be between the two goals at about the level of the goals.  There are three goal hoops, fifty feet high, at each end of the stadium.  IV-96  There is no apparant limitation as to how high the players can go.
 

Equipment

Brooms
Quaffle - Scarlet, about the size of a soccor ball, this ball is used to score
Bludgers - Black heavy balls slightly smaller than the Quaffle, they can injure a player who gets in the way.  The Beaters keep them off the other players and try to direct them at the opposing team.
Golden Snitch - This small, winged, gold ball is the size of a walnet, is elusive and hard to spot.  The Seeker capturing it earns his team 150 points and it ends the game.  Usually the team capturing the Snitch will win the game, but not always.

Officials
One Referee on broom with whistle.  He enforces the rules and assigns penalties.
 

Rules:

The game begins by the Referee releasing the four balls and blowing his whistle.  A team scores ten points for succesfully getting the Quaffle through one of the goal rings.  The game ends once the Golden Snitch has been caught.  The longest game on record is one of abouut three months.  I-169

Substitutions of players is allowed, though seldom done unless the game drags more than a few hours.

Penalties -
There are 700 ways of committing a Foul in Quidditch.  All 700 occured in the World Cup match of 1473.  I-181  Most of them are no longer possible given the ban on the use of wands by players and the audience.

Those we have seen specifically called are:
Cobbing - Excessive Use of Elbows - Penalty Shot - IIV-109
Delibrately trying to run into an opponent - IV-111
Penalties can be awarded due to audience or mascot misconduct.

Standard Strategies:

Hawkskead Attacking Formation IV-106
Porskoff Ploy IV-106
Wronski Defensive Feint - Dangerous Seeker Diversion (IV-109)

Quidditch is played on three levels.  The intermural level of school competition between houses, the professional level, run similarly to the soccer leagues in England, and the international level of the Quidditch World Cup.

International Teams We are aware of
Bulgaria - Runners up in 1994
England - Lost to Transylvania
Ireland - Winner of the 1994 Quidditch World Cup
Luxemburg - Beat Scotland
Peru - Lost to Ireland
Scotland - Lost to Luxemburg
Transylvania - Beat England
Turkey
Uganda - Beat Wales
Wales - Lost to Uganda

Professional Teams in other Countries
Moutohora Macaws - New Zealand - Red, yellow and blue robes and their phoenix mascot Sparky.
Thundelarra Thunderers - Australian League
Woollongong Warriors - Australian League
Bulgarian Vratsa Vultures - Seven times European Cup winners
France - Quiberon Quafflepunchers shocking–pink robes
Germany - Heidelberg Harriers
Luxembourg, -Bigonville Bombers
Portuguese - Braga Broomfleet
Polish Grodzisk Goblins - Seeker, Josef Wronski.
Patonga Proudsticks - Uganda
Tchamba Charmers (Togo), masters of the reverse pass
Gimbi Giant–Slayers (Ethiopia), twice winners of the All–Africa Cup
Sumbawanga Sunrays (Tanzania)
Moose Jaw Meteorites - Canada
Haileybury Hammers - Canada
Stonewall Stormers. - Canada
Bulgarian Vratsa Vultures - Seven times European Cup
Quiberon Quafflepunchers - France
Sweetwater All–Stars - Texas
Quiberon Quafflepunchers
Fitchburg Finches - Massachusetts - Won US League seven times
Tarapoto Tree–Skimmers - Peru
Toyohashi Tengu - Japan
Gorodok Gargoyles - Lithuania

There are 13 Professional Teams in Great Britain

Appleby Arrows - Robes - Pale blue, emblazoned with a silver arrow
Chudley Cannons - Robes - Orange with two black Cs and a Cannonball
Ballycastle Bats - Black robes - Scarlet bat on front - Mascot Barny the Fruitbat
Caerphilly Catapults - Robes - Vertically striped light green and scarlet
Falmouth Falcons - Robes - Dark–grey and white - Falcon–head emblem on chest
Holyhead Harpies - All Female team - Robes - Dark green - Golden talon on chest
Kenmare Kestrels - Robes - Emerald–green - Two yellow 'K's back to back on chest
Montrose Magpies - Robes - Black and white - Magpie on chest and back
Pride of Portree - Robes - Deep–purple - Gold star on chest
Puddlemere United - IV-84 - Robes - Navy–blue - Emblem - two crossed golden bulrushes.
Tutshill Tornados - Robes - Sky–blue - Double 'T' in dark blue on chest and back
Wigtown Wanderers - Robes - Blood–red - Silver meat cleaver on chest
Wimbourne Wasps - IV-86 -IV 524 - Robes - Yellow and Black Striped - Wasp on chest.