Grant and Lee were both Civil War generals, but in addition to being on opposing sides of the dispute, they had very different backgrounds and military experience. There were major differences in their styles of wartime strategy, for instance. Lee was a natural leader who was able to inspire his men, while Grant was able to effectively use his superior resources and manpower.
Lee was born into a prominent family and was already a famous military figure at the start of the war, while Grant was an unknown who rose up through the ranks and became known as a great general through his ongoing victories. Lee was a traditional soldier who had a gift for leadership and was an inspiration for his subordinates.
Through respect and admiration, Lee was able to get his men to do the impossible with very limited resources. Grant was not particularly clever in his military tactics, but he was good at using his access to Northern resources to a great advantage. His strategy was to overwhelm Lee and the Confederate army, and his tenacity enabled him to eventually wear down the South. Although history considers them to both be great generals, Grant was able to methodically use resources and strategy to outmaneuver and ultimately defeat Lee over the course of time.