Aleppo is the largest city in Syria and its commercial capital. It is to the political capital, Damascus, as New York is to Washington, Shanghai is to Beijing, Mumbai is to Delhi, Toronto is to Ottawa, Milan is to Rome, Amsterdam is to The Hague, and I could go on. However, few know of it, and it has all but disappeared from the western media. But, now that the US is entering into the Syrian conflict, it behooves people to pay more attention to it, for the idea that the US can build up the Free Syrian Army, or any "moderate" secular opposition that will not introduce Shari'a if it overthrows Assad after defeating ISIS/ISIL, depends very much on who controls Aleppo.
The quick answer is that as of now, nobody does. It is split nearly in half between government control, mostly in the south and west, with "rebels" controlling in the north and east, although the zones of control are not separated by a neat line, with a small area controlled by Kurdish forces.
In terms of the what Obama says he wants and is being pushed by Sunni Arab allies such as Saudi Arabia and Qatar to do, it becomes important who the non-Kurdish "rebels" are. This is a very complicated matter, and who they are has changed over time. However, as of now, it looks like by far the largest group among them is the Liwaa al-Tawhid Brigade, a group that at one time was allied with the Free Syrian Army, the group that Obama favors. However, last fall it declared that it supported imposing Shari'a law and has joined the Syrian Islamic Front, which originally had 11 groups in it, but is now down to 7, with those most closely linked to al-Qaeda, such as Nusrat, leaving the group. ISIS/ISIL (and the newly publicized Khorezan) never belonged to this group, which has pushed them out of Aleppo.
It must be kept in mind that there has been near constant war in Aleppo since 2012. This was the period when neocon critics claim Obama could have tilted things against Assad and ISIS/ISIL if he had supplied more aid to the Free Syrian Army. Maybe. But what is striking if one looks at a detailed history of the Battle of Aleppo, is that there have not been any major shifts of control since the battle started, although there have been minor shifts here and there. It should also be kept in mind that throughout, most of the Christians in Aleppo have supported the Assad government against all the rebel forces. Christians did not do well in Iraq when a US-backed Muslim sectarian regime took over, although there a Shi'i one rather than a Sunni one that would take over in Syria.
It is al-Tawhid that is the most serious non-ISIS/ISIL group in Aleppo, and they are Islamist, supporting the imposition of Shari'a law, if they win. The only larger group in the Islamic Front is Ahrar al-Sham, a hardline Salafist group more radical than al-Tawhid, although they are mostly operating in other parts of Syria. Obama is kidding himself if he thinks that aiding the "moderate" Free Syrian Army is going to take ground from these strong Islamists, much less ISIS/ISIL. At most, the FSA controls only about 20% of the anti-Assad rebel forces.
I suspect that Obama feels pushed by the neocons to try to support the Free Syrian Army, with the videos of the beheadings by ISIS/ISIL pushing him as his polls have collapsed. As it is, the Saudis and Qataris are fine with the gainers being the Islamic Front, which they are backing.
For an account of the Islamic Front and its sub-groups, see the excellent report by Aaron Lund for the Carnegie Endowment. For an exhaustingly blow by blow account of the Battle of Aleppo, Wikipedia has a pretty good account under that name.