Paris is divided into 20 arrondissements, or districts. The first arrondissement is in the middle of the city, then they spiral out like a snail shell. Each has it’s own distinct character and they can differ widely from each other. Paris is also divided into Right Bank (north of the river) and Left Bank (south of the river). The Right Bank is traditionally the area of commerce and business, the Left Bank of learning, although now they are more mixed.
Every arrondissement is worth a visit, some more than others of course depending on your interests, and seeing a bit of each gives you a good overview of the patchwork of neighbourhoods that makes up the city.