In his depiction of the jurors, Rose has chosen a cross-section of American society. This is the way juries are selected. It is a long and arduous process that he is careful to represent. The jury itself is comprised of a variety of people; the educated, the old, members of the working-class, business men, and even immigrant men. These choices are not based on the individuals themselves, but rather their social representations. Rose's purpose in this is to represent a "trial by one's peers."
Examples:
The foreman is a football coach; Juror 2 works in a bank and is rather shy; Juror 3 owns a small business; Juror 4 is a stockbroker; Juror 5 works in a hospital (he can also relate to the defendant because he was raised in the slums); Juror 6 paints houses; Juror 7 is totally disinterested in everything; Juror 8 is an architect (he's also a logical, deep thinker and ready to stand up for what he believes); Juror 9 is an old man (reason and experience); 10 is a bigot; 11 an immigrant (determined to do his duty in a country he embraces); and 12 works for a marleting agency.
As you can see the jury comes from a number of diverse backgrounds. They influence each other and represent the American people. These men play off of each other and use their experience to make one decision.
If you need any further information on the individual jurors see Gradesaver character analysis linked below.