While there are quite a few differences between the circulatory systems in the human and the frog, the primary difference between the two hearts is the number of chambers; frog hearts have two atria and one ventricle, while human hearts have two atria and two ventricles. The frog's right atrium gets deoxygenated blood out of the vessels that come from the bodily organs, and the left atrium receives oxygenated blood from the skin and lungs. Both of the atria send blood into the ventricle which has two chambers that keep the two sources from mixing.
The ultimate destination for oxygenated blood within a frog is the carotid arteries, which send blood to the brain, while the deoxygenated blood goes to the lungs and skin to gather oxygen.
Within a human, circulation forms a loop from the heart throughout the different sections of the body. Arteries gather blood with oxygen from the heart and send it out to the tissues where it disperses oxygen and gathers carbon dioxide on the way back to the heart. Then, the blood goes from the heart to the lungs to receive oxygen before coming back to the heart and then out through the primary arteries again. The pulmonary arteries and veins link the heart to the lungs.