How do Sampson and Gregory get into a fight with Abraham? Who is Abram?

Gregory and Sampson are servants from the house of Capulet. In the opening scene of Romeo and Juliet, the two are walking down a public street in Verona. While walking, they are joking about how they would challenge men from the house of Montague and would, essentially, rape Montague women. As reported in the play's prologue, there is an "ancient grudge" between the two wealthy families. Gregory suggests that the fight is not only between their masters, but also "us their men."
When Abram, a Montague servant, and another "servingman" pass by, Sampson says he will bite his thumb at them," which is a disgrace to them if they bear it. When Abram questions whether Sampson is biting his thumb with aggression, Sampson retorts, "No, sir, I do not bite my thumb at you, sir, but I bite my thumb, sir." Eventually, in the heat of the moment, swords are drawn and fighting begins. When Benvolio, a Montague and cousin of Romeo, attempts to stop the brawl, Tybalt, Juliet's cousin, intercedes, threatening to kill Benvolio.
Thus, from a simple insult directed at Abram, associates of the two warring families stage a violent showdown in the street which results in a visit from the city's prince and a swift rebuke toward Lord Capulet and Lord Montague. The prince declares that further fighting between the families will result in the death penalty being levied on the guilty party or parties. This threat, of course, will have grave consequences later in the play.
The scene is excellently portrayed in Franco Zeffirelli's sumptuous filming of the play from 1968. It is made even more dramatic by utilizing colors to distinguish the two sets of servants, with the Montagues wearing dark colors and the Capulets dressed in bright reds and oranges. In contrast, Baz Luhrmann's version from 1996 places the scene in modern times with cars and guns. It ends with an explosion at a gas station. Interestingly, the colors from Zeffirelli's version are reversed, with the Montagues now dressed in bright colors and the Capulets wearing dark. Both movies do a good job of depicting the enmity between the Montagues and the Capulets.