Good morning, all...
The FA Cup is a competition arranged by the Football Association (aka, the FA) to include all levels of professional football in England. So long as you are a registered professional team in England and meet and abide by the standards set forth by the FA, you can submit an entry into the FA Cup. It is the ultimate knock out competition in world sport. Who knows, someone from the football conference (below division 3) could get to the 4th, 5th or 6th rounds.
The competition starts in early August with the extra preliminary round, then a preliminary round, 4 qualifying rounds which then creates the competition proper. I believe the winner in the preliminary round receives 75 pounds or something.
The 1st round proper introduces the 3d division teams, the 2d round introduces the 2d division teams and the 3d round introduces the division 1 teams and the Premiership.
In all rounds through the 6th round, if the game ends in a draw, there is a replay and the venue switches. For the next FA Cup, the replays will be eliminated after the 3d round proper (games will go into AET and spot kicks if necessary). In the past, all games that ended in a draw after 90 minutes went to a replay, including finals (see United's 1990 triumph over Crystal Palace, 3-3 in the first match, 1-0 in the replay).
The match for United, while quite less important than the Premiership and the UCL, is meaningful because it allows United to claim some silverware this season and participate in the Community Shield in Cardiff next year against Arsenal. Furthermore, because of United's failure in the UCL, the game had financial implications for United because winning the game allowed United to claim upwards of 6-10M pounds and allowed them to nearly match what Arsenal bagged from an extra round in the UCL.
Furthermore, Millwall, by being the finalists, secured a UEFA Cup spot, which squeezed out Aston Villa from European football in 2004-05 (the FA Cup finalist gets a UEFA Cup spot and 'Boro claimed the other by winning the Carling (league) Cup in January).
The Gunner/United game at Villa Park was great theatre, but the best match of the 2004 FA Cup was the 4th round between Spurs and Man. City. Spurs went up 3-0 at White Hart Lane, but City went a man down (sent off) at the stroke of half time. City, with 10 men, scored 4 in the second half to beat Spurs 4-3. City were then beaten in the 5th round at Old Trafford by 10 man United 4-2 when G Neville got sent off after a dive in the area and heat butting MacManaman.
At least United went 2-2-0 against Arsenal this past season (win in the Comm Shield and in the FA Cup Semi, 2 draws in the premiership).
Lastly, the Gunners weren't undefeated; a title like that would be reserved to the 1972 Dolphins. I call what they did, not only remarkable, but non-defeated.