The Loop is the business and financial center of Chicago. You can see a ot of high buildings there of which the Willis tower is.
The Loop is near Lake Michigan. It is what people call "Downtown Chicago" or "Central Chicago" which is one of the four section of Chicago territory.
It is the 2nd biggest financial district of the U.S. after Manhattan in New York.
This district is called the Loop because of the Chicago Surface Lines which forms a loop in the district.