There are two main versions of the Bent Over Row - 45 degree Bent Over Row and 90 degree Bent Over Row.
45 degree can be either supinated or pronated, supinated places more emphasis on the biceps. It is significantly easier than the 90 degree, after getting used to this exercise you should be able to do the same weight and reps that you would do on the bench press. This exercise emphasizes the lats, teres major, biceps, rhomboids, some middle and upper traps but not that much as was stated earlier. Some rear delts but not that much emphasis on them either.
The 90 degree Bent Over Row is a much harder version. This is done with a wider grip, almost always pronated. You want to flare your elbows out as your row, basically like doing a bench press facing the floor. The bar should hit your lower chest, on the 45 degree it should hit your belly button or the waistline of your shorts. This hits the lats and teres major, rhomboids and middle traps, the rear delts and since it is pronated more brachialis and brachioradialis. The pronated wide grip puts more emphasis on the upper middle back (rear delts, middle traps, rhomboids) while a supinated narrow grip places more emphasis on the lats and biceps. The 90 degree also places much more stress on the lower back, for that reason (and just because it is harder) some people don't like it, but both are great exercises. Similar to comparing a chin-up (with a short range of motion) and a widegrip T-bar row.
Hope that clears it up for you.