Hog Butcher for the World,
Tool Maker, Stacker of Wheat, Player with Railroads and the Nation's Freight Handler; Stormy, husky, brawling, City of the Big Shoulders:
The poem begins in a pretty intense way. The speaker personifies the city (gives it human qualities) by describing it as a "Hog Butcher." Note that the speaker doesn't say that Chicago "is a city where Hog Butchers live," or even "a city of Hog Butchers." He just gets right to it, and calls the city a Hog Butcher, thus bringing Chicago to life. It's as if the city itself lives and breathes (and butchers too).
And notice that "Hog Butcher," as well as all the other names for the city, are capitalized, as if they are titles (like "Doctor," or "Professor"). This has the effect of elevating these seemingly-low titles. (Have you ever addressed a letter to "Hog Butcher Smith?" We think not.) And this is a pretty violent way to begin the poem, don't you think? We might expect a poem that glorifies a city to begin by celebrating its beauty, but Sandburg doesn't whitewash anything for us. Chicago is a tough place where a lot of hogs are butchered, thus the poem begins with swine and blood. Just deal with it, Sandburg says.
Then the speaker goes on to describe other attributes of Chicago in a similar way. It's a city known not just for butchering, but also for its harvesting of wheat, its industry, and its importance to the cross-country railroads. (And Sandburg himself worked on the railroads as a teenager.) Again, no beauteous skylines here. Chicago is a place for laboring.
Then the speaker describes the city to us with adjectives (instead of with occupations). It's "Stormy, husking, brawling," a "City of the Big Shoulders." To say that this is a city of big shoulders is to use synecdoche, using a part to describe a whole. In this case, the speaker uses a very small part (a part of one person: the shoulders) to describe a whole city. Still, this is a very evocative move (which may be why "The City of Big Shoulders" remains a popular nickname for Chicago). We can imagine a broad, big-shouldered person, domineering and assertive. By extension, we can relate that personality to the city as a whole.
Your personification-meter should really be flashing now. It sounds like the speaker is telling us about a tough muscle-man now, even though he's still talking about the city (which is filled with tough muscle-men).