Both versions are on the US-DVD (NTSC):
Theatrical Cut: 154:51 minutes
Extended Cut: 170:47 minutes
The Extended Cut includes 14 additional scenes, which is a total of 15.56 minutes of extra footage.
Publishers always try to find new ways to squeeze money out of movie fans. That`s why almost every Hollywood movie is released in more than one version. And Ridley Scott's Gladiator is not an exception to the rule.
For the Extended Cut, about a dozen scenes were integrated into the original Theatrical Cut of this famous historical drama.
However, those scenes were already on the original DVD as deleted scenes. Furthermore, Ridley Scott does not seem to be very happy with this new Cut. In a short intro, he emphasizes that the Extended Cut is not the Director`s Cut. Scott personally prefers the Theatrical Cut because he thinks that although the new scenes are somewhat interesting they are definitely not necessary.

17:29
Maximus visits injured soldier in a field hospital after the battle in Germania libera.
69 sec

31:48
Shortly before Marcus meets his son to tell him that he is going to be emperor, he prays to his ancestors.
21,5 sec

52:40
One of the new gladiators, who used to be a scholar, is scared and asks Hagen what he would have to do in order to be freed.
Hagen explains to him how many men he would have to kill for that. The scholar is shocked and, unlike Hagen, he does not want to go that far.
Here, the TC shows a different shot, where the black guy walks toward Maximus
58 sec longer

53:42
In the market, Christians are shown. Those are then sent into the arena together with lions.
Furthermore, Proximo talks to his colleagues about gladiators and tries to convince them to bet against his own.
81,5 sec

66:24
Proximo tells Maximus before his next fight, which he has to fight alone that the audience expects to see a hero and not a butcher.
Maximo has to entertain the masses.
12,5 sec

66:35
Proximo tells Maximus, before he enters the arena, one more time that he first of all must be entertaining
2 sec

77:39
Lucilla meets the senators Graccus and Gaius. They are all worried about the future of Rome and wonder what Commodus is planning. The games are too pricy and are financed by selling off their corn reserve, which would lead to severe famines in about two years. The problem is that no senator dares to address that issue because they fear to end up in the arena themselves. Lucilla says that Commodus has to die. The senators are against it because they fear the praetorians. Therefor Gracchus suggests to wait until Commodus has so many enemies that a strike could be successful.
233,5 sec

93:59
Commodus goes into the basement of the palace, where spoils of war are said to be stored. When he comes across the bust of his father he draws his sword and attacks it to release his frustration. After that he wants to punch it but changes his mind and embraces it out of despair.
99 sec

99:28
Commodus has two praetorians who knew about Maximus' escape put to death to test the loyalty of Quientus. Lucilla watches the spectacle in which Commodus walks in front of the archers, who have already drawn their bows. This show that he has lost his mind.
While the archers are already trembling, because they are not allowed to shoot, Commodus continues questioning Quintus' loyalty. He then walks between the two praetorians, who are then executed.
167,5 sec

112:41
After Falco has told Commodus that Maximus has to die and Commodus has denied that, Commodus tells the senat that he has to sell of the corn reserves in order to finance the games and that he is surprised that there is no response from the senators.
The first part about Maximus death is shown in the TC in a different shot.
25,5 sec longer

118:04
Gracchus sends his servant to Proximo in order to pay for Maximus' freedom. Proximo realizes that the servant and he himself are watched by Commodus' spies and blows the deal.
71 sec

119:47
Lucilla realizes that her brother and Falco are allies.
54,5 sec

129:45
When the praetorians enter Proximo's house, they put some of the servants on fire
18,5 sec

136:07
Shortly before Maximus' last fight, Quintus talks with him to calm his own conscience. Maximus, however, rejects him.
29,5 sec