Lebanon has had a continuously existing Christian population pretty much since the foundation of Christianity. There are 11 different confessions of Christians in modern day Lebanon, the largest group are the Maronites whose history goes hand in hand with Lebanon. During the Muslim conquests, the Maronites retreated to the Lebanese Mountains (which are the highest in the Middle-East I believe) where they were mostly safe from persecution. The Maronites were isolated from the rest of Christendom for several centuries, only renewing ties during the Crussades, at which the point the East-West schism had happened. However the Maronites swore fealty to the pope in Rome and basically became a catholic church (separate from the mainline Roman Catholic church) despite still retaining many characteristics of eastern churches, the most obvious example being that Maronite priests are allowed to marry (but if they do so they can no longer become bishops or hold certain other positions). Lebanon was pretty much created for the Maronites who at the time of the country's foundation still constituted the majority of its population (this statement may be somewhat controversial as this was not expressly stated at the time of modern Lebanon's creation but was heavily implied).

The second largest group are the Greek Orthodox, historically they mostly lived in the cities along the coast and co-existed with Muslims, some of them are remnants of the Byzantine empire, others decedents of Greek immigrants, etc...

The third group are the Greek Catholics (Melkite), they are basically an offshoot of the Greek Orthodox who reconciled with Rome and entered into communion with them during the 19th century. They are mostly concentrated in the East of the country in the Bekaa valley with some others in the South and Beirut, there are virtually none in the North for some reason.

The fourth group is Armenians (orthodox, catholics and protestants), all decedents of refugees from the massacres perpetrated against the Armenians by the Ottomans in the early 20th century. They are technically Lebanon's only ethnic minority, although you can't really tell them apart just from looking at them, but they do have somewhat different traditions, still largely use Armenian rather than Arabic (they will often not be very proficient in Arabic) and tend not to mix with the other groups as much. Their numbers have been dwindling rapidly in the past few decades as they have emigrated in larger numbers than other communities.

The other groups include Roman Catholics, mostly decedents of French and Italian emigrants. Protestants, mostly converted during the 19th and early 20th century. Syriacs, Copts, Chaldeans, all of whom are descended from emigrants and refugees from surrounding countries. These communities only number a few thousand each, I (having lived in Lebanon for most of my life) have never personally met any Lebanese Syriacs, Copts or Chaldeans (although I know of some), I've only met one or two protestants and a know a couple of Roman Catholic families (Maltese and Italian descendants).