I am hearing a lot about the persecuted Christians in Aleppo in Syria, I would like to know about the importance of the community, it's history and if they are related to the Crusades.

Christians made up about 10% the (pre-War) Syrian population, though some estimates say that it may have dropped due to migration and been closer to 8%. This population is disproportionately concentrated in major cities like Aleppo, Homs, and Damascus, but there are Christians villages and regions outside of major cities. Mostly, the Christian denominations of Syria are not related to Crusaders. I will try to go by the group by group, but I am not sure of the actual relative population sizes. As a State Department Religious Freedom Report for Syria says, "It is difficult to obtain precise population estimates for religious subgroups due to government sensitivity to sectarian demographics." I include a wikipedia list of church buildings in Aleppo ascribed to various at the end. The largest group in Syria is the Greek Orthodox church, who I believe are largely Arabic speaking today (unlike, say, the small population of Greek Orthodox left in Istanbul, who still use Greek as a home language) though they continue to use Greek as a liturgical language. Remember that the whole region was ruled by the Christian Byzantine Empire (and occasionally the Persians) before the Arab conquests, so the whole area was once majority Christian, largely Greek-speaking, roughly 1300 years ago. These Arabic-speaking Greeks are the largest remnant of that Byzantine/Eastern Roman population. Which is to say, they are not for the most part connected to the Crusaders who came, at least nominally, to reconquer Jerusalem from the Arabs after the Byzantines had lost it/defend the Byzantines (the various crusades had a variety of goals, that these people often didn't live up to). The seat of this church, interestingly, is not in Constantinople or Rome, but in Antioch (modern day Antakya, in southern Turkey, right at the little corner where the Levant meets Anatolia). Confusingly, the name of this See is ceremonial and as far as I can tell this Patriarch actually sits in Damascus. I can't tell when this tradition started, but I wouldn't be surprised if it originates in the 20th century.

In the 18th century, this church split into two, essentially. A part stayed on affiliated with Orthodox Church as mentioned above (where the Patriarch of Antioch was one of the fourteen or so autocephalous hierarchs of the Eastern Orthodox Church, though there were traditionally fewer) and one group split due to an internal dispute and affiliated themselves with Rome. This group is today known as the Melkite Greek Catholic Church. This wasn't a rare thing in this period, and it's basically how many of the Eastern Rite Catholic Churches formed (the Maronites are a notable exception). They practice a Greek Rite (liturgy) similarly to Eastern Orthodox Churches, but they are in full communion with the Roman Catholic Church. I believe Melkite history claims that they were always in communion with Rome (that is, they did not recognize the formal split in 1054), and the history is very complicated (starting in the Crusades there was an clear Latin presence in the area, which I believe many would argue form one of the bases of the Melkite Catholic Church, but I'm not sure if this presence was really continuous) and it's something I don't know particularly much about, unfortunately. This is generally seen as the second largest group in Syria as a whole, only slightly smaller than the Greek Orthodox Church.

The next largest groups are likely the various Syriac liturgies, though I believe they have a large presence in the north than in Aleppo. I have a longer post about the history of the Assyrian Churches that you will likely be interested in. Since it was focused on Syriac as an ethnicity and not Syriac as a liturgy, it only briefly mentions things like Syriac Catholic Church (which is based in Beirut) and the Maronite Church, which are two separate Eastern Rite Catholic Churches that use Syriac liturgies and whose members--as far as I know--tend to think of themselves ethnically as Arab or Phoenician, but not generally Syriac. I'm not particularly familiar with the history of Syriac Catholic Church, but it seems like many other Eastern Rite Churches, it dates to the 17th century (though, again, Catholics had been a presence in the region for longer) and had a spotty history until the 18th century. The Maronite Church is dominant in Lebanon, but much smaller in Syria. Maronites tend to describe themselves ethnically as "Phoenician" or "Arab", which often indicates a political orientation. The Maronite Church has a very interesting history--one that I admittedly don't fully understand. Suffice to say they predate the Crusaders, but established ties with them and quickly declared themselves in communion with Rome (a communion that has been unbroken since). I unfortunately have no idea the relative sizes of these groups that use Syriac liturgy.

The last large group is the Armenian groups. The largest Armenian group is the Armenian Orthodox Church (which is an Oriental Orthodox Church, like the Syriacs, not an Eastern Orthodox Church in common with Constantinople). These have nothing to do with the Crusades, and indeed split off from the Catholics/Eastern Orthodox in the 5th century (Catholics and Eastern Orthodox formally split in 1054, as mentioned above). There are likely also Armenian Catholics (another Eastern Right Catholic Church) and Armenian Protestants (converted largely by American missionaries working in the Ottoman Empire). There is a long history of Armenians in the region, but the community greatly expanded with refugees from the killing and expulsion of Anatolia Christians (commonly known as the Armenian Genocide, but this event also targeted other sorts of Christians). This famous map shows, for example, that the end point for many survivors was the Syrian desert near Der Zor (also called Deir ez-Zor). This expulsion greatly increased the number of Armenians in Syria and it also expanded the number Syriac Christians who used to live in south Turkey (especially around Mardin). Aleppo I'm told has a particularly large Armenian population.

Beyond this there are also Roman Catholics (that is, Western Rite Catholics), whose history I don't know--I would not be surprised if at least some part of the organization dated back to the Crusades though some are also likely recent converts--and Protestants, who are recent converts. As with the Armenian Protestants, many are presumably converts from other sects of Christianity during the Ottoman Period (the Ottomans allowed Protestant and Catholic missionaries, but they were only allowed to proselytize to non-Muslims) though some are likely post-Ottoman converts from Islam. The Wikipedia article for the Apostolic Vicariate of Aleppo seems to indicate that the continuous Roman Catholic presence in the region is a distinctly post-Crusades phenomenon.

So, in short, the vast majority of Syrian Christians are not related to the Crusaders, but are part of churches that have much longer roots in the region (this is a region where only 800 years can seem relatively short). Lastly, I found a list of churches specifically in Aleppo. According to that list, there are four Armenian Apostolic churches, four Armenian Catholic, four Armenian Evangelical, one Syriac Protestant, three Syriac Orthodox, three Syriac Catholic, five Greek Orthodox, seven Greek Catholic, one Chaldean Catholic, two Maronite, seven Roman Catholic, and two Protestant churches (one Presbyterian Church dating the Ottoman Empire and a more recent one presumably related to more recent evangelizing). Even if this list is accurate as of the start of the Syrian Civil War, these are the number of buildings and shouldn't necessarily be taken to be indicative of relative population sizes.