Many factors led to Northern victory, but for the purposes of this answer we should focus on the two I believe are the most important. First, as has long been recognized, the Union was far better off than the Confederacy in terms of its industrial strength and its manpower. The South had few factories, and its economy was primarily agrarian. The industrial facilities and larger population of the North made the Union self-sufficient, while the South was dependent on its ability to sell agriculture products—principally cotton—to the European powers in exchange for the armaments and other supplies it needed to keep the war going. The Union early in the war imposed a naval blockade that prevented the unimpeded trade needed by the South to fuel its military effort. Blockade runners, though they were partly successful, could not make up the deficit, and the capture of the few remaining open Southern ports late in the war made Northern victory inevitable.

Second, though it hasn't always been recognized by historians, it was arguably the North's superior military leadership in Ulysses S. Grant and William T. Sherman that accounted as well for its ultimate victory. Both tactically and strategically, Grant overcame enormous obstacles in the Western theater, culminating in the Vicksburg campaign, which led to Union control of the Mississippi river by July 1863. When made supreme commander early in 1864, Grant, together with Sherman, devised a unified strategy that cut the Confederacy in two. Though taking enormous casualties in Virginia, Grant was able to slip his forces around Lee and cross the James river, leading to the siege of Petersburg, trapping Robert E. Lee's Army of Northern Virginia and making Southern victory impossible. At the same time in the deep South, Sherman out-maneuvered Joseph Johnston's forces, took the railway hub of Atlanta, and demoralized the South by destroying huge amounts of property in Georgia and South Carolina.

The South's weaknesses included its inability to achieve recognition by foreign governments. Though Great Britain and France initially gave a kind of tacit support to the Confederacy, even this was withdrawn once Lincoln issued the Emancipation Proclamation and made the liberation of the enslaved people the purpose of the war. The fact that the slavery issue was the cause of its secession—which Southerners in the aftermath of the war and for decades afterward attempted to deny—was essentially what doomed the Confederacy. In addition, there was no unified strategy behind the South's military efforts. Little was done to coordinate the campaigns in the Eastern theater with the West. As a Virginian, Lee , it can be argued, overly prioritized his own state. His campaigns were also overly aggressive, in the judgment of many contemporary historians, and he took enormous casualties he could not afford to take, even in battles he won. Yet part of the reason Lee did not engage in a more defensive strategy was that, given the South's need to protect slavery, he could not afford to give up territory to the Union forces. Thus, both of the principal weaknesses we've described stem from the slavery issue that motivated the South to seek independence to begin with.

I have to agree with the main points of the previous posts in that the North's superiority of numbers proved to be their strongest asset. Militarily, the South held the advantage at the beginning of the war. Their cavalry was far superior for much of the war, and Southern leadership at the upper ranks was also at a higher level. However, as the war dragged on--and with it mounting casualties--it became impossible for the South to replenish their ranks. In the North, the population was far larger in numbers, and the nation continued to grow through immigration during the war years. When the call came to add Negro troops, it further enhanced the Union's military numbers.

The Federal navy was far superior to the Confederacy's at the beginning of the war, and it only became stronger as the war progressed. Eventually, the Union cavalry became an even match for their Southern counterparts. When Lincoln finally discovered great commanders (Grant, Sherman, Sheridan, Thomas) at the highest levels, it was only a matter of time for their numerical superiority in all branches to overwhelm the Confederate forces.

Northern strengths and Southern weaknesses in the War between The States:

Northern strengths include a large industrial base, a larger population from which to draw both soldiers and industrial workers, soldiers recruited from Europe with promises of land (the Pope issued a letter opposing this), black soldiers (many volunteers and many impressed), a central authority (Abraham Lincoln) that could and did imprison thousands of citizens who opposed the war, a war leader (Abraham Lincoln) who was good at working with other politicians and who finally became able to turn the fighting of the war over to his generals.

Southern weaknesses include a small industrial base, a smaller population, a resistance to using blacks as soldiers (resistance on the part of politicians; generals enrolled tens of thousands of black soldiers, but not the hundreds of thousands that the North enrolled and that the South could have enrolled but for political opposition), a weaker central authority so that some state governors (notably Ga., and N.C.) did less than they could have done to cooperate with the national leaders of the war effort, a war leader (Jefferson Davis) who was not good at working with other politicians and never became able to turn the military side of the war over to military men.

All of the previous posts were very much accurate.  I would add my own opinion to the mixture by suggesting that the mentalities of both sides helped allow for victory and set the stage for defeat.  The North understood that they had a greater chance of winning if they could extend the war, use their industry to end up defeating the South through attrition, and allow their superior numbers in manpower to take over.  This mentality allowed them to weather through some early defeats and setbacks in order to embrace the larger goal of victory.  The Southern mentality played into this.  They figured that the conflict would be a quick one, a relatively simple affair where their superiority in training and war fighting tradition would scare off the Northerners.  It seemed that the Southern mentality which embraced a quick win and a Northern one that embraced a prolonged conflict were complements to one another, and a set that played out in a Northern victory and a Southern defeat.

The greatest advantage the Union had in fighting the Civil War was definitely population.  The Union had about 22 million people while the Confederacy had about 9 million, with over 3 million of those people being slaves.  When Ulysses Grant took over as commander of all Union forces, he realized the potential of this population difference and began fighting a war of attrition.  He engaged the enemy whenever he could.  He knew that he could replace his losses while the Confederacy could not.  This was a brutal and bloody strategy, but it worked.

Perhaps the greatest weakness the Confederacy had was a lack of any major European ally.  As the war began, southern leaders felt that Great Britain would join them in their fight against the Union.  The Confederate leaders believed this because they thought that Britain would need their cotton for the British textile industry.  If Britain had joined with the Confederacy, then the Confederacy would have had more fighting men, a navy, an industrial base in Britain, and more money to fight the war—perhaps completely changing the outcome of the war.

The major Northern strength that led to victory for the Union was the North’s huge advantage in population and industrial power.  The corresponding Confederate weakness was a major factor in its defeat.  Going into the Civil War, the North had a tremendous advantage in population and industry.  As the link below tells us, the North manufactured 90 percent of the boots and shoes produced in the country.  It produced 93 percent of the country’s pig iron and 97 percent of its guns.  The North was home to almost five times as many free people as the South.  All of these things were huge advantages for the North.  These factors gave it more men to throw into the war and more weapons, boots, and other things with which to equip them.

There were some other Northern strengths that mattered to some degree.  The North had a navy while the South really did not.  This allowed the North to blockade the South, helping the North to win the war.  A Southern weakness that helped cause the Confederacy to lose was the fact that it was a confederacy.  The Southern states did not want to allow their national government to have much control over them.  This led them to do things like resisting putting up the money the national government needed for the war.

The most important factors, though, were population and industrial capacity.