Presbyterian is a christian assembly, a product of the protestant reformation.
This particular branch of Christianity (in general) holds to reformed theology which, in turn, offers the most accurate system of interpretation in my opinion. John Calvin is the founder of the Presbyterian assembly.
But on the other side of the coin there is a "liberal" branch and a "conservative" branch, like in most organized assemblies, and, through the liberal branch, world pollutions and heretical practices are creeping into their congregations. Such practices and doctrines to which the ascribe are derived neither from scripture nor reformed theology but, instead, from false teachers within the church's hierarchy.
if you have accepted Jesus Christ as your lord and savior and confessed that he is lord, then you are a Christian. Whether you are Presbyterian, Catholic, Baptist or what ever religious label you choose to study under, you are still a Christian. We are all part of the body of Christ.
The Presbyterian denomination is one of the four Protestant churches which objected to doctrine of the Roman Universal Catholic Church lead by a Frenchman John Calvin in Switzerland and John Knox in Scotland. It came out of the umbrella of the Roman Catholic in the 1500's AD, becoming the State Church of Scotland. It remains much as its former self much like the Catholic, is questionable at times as to whether its organization is Christian while many of its members seem so.
The primary difference of Presbyterian and Christianity is Christianity was founded and formed by Jesus Christ and follows a very orderly line of doctrine. The Presbyterian comes long after its founding organization the Roman Universal Catholic Church which was founded by Constantine the Great in AD 313. It was his hope to unite the various rebellious Roman townships by forming a common state Church---It's basic organizational structure is of the Mithra Pagan Religion of Babylon of 600 BC. It has never practiced the 13-15 doctrinal steps for Jesus' church so is hardly a Christian organization. Many Catholic people have become Christian while its organization is still basically Pagan. So that is to say the four Protestant groups who protested against the Mother Church and ultimately split away are within themselves still very much Catholic in their organization and worship and surely her doctrine. The head of the Christian church is Jesus, the head of each of the Protestant ultimately is the high ranking member of their organization and that of the Catholic the Pope. The Christian church each stands alone with very few exceptions. They have no boss or organizational structure to yield to, the Methodist being surely one exception.