Welcome to the Forum brother.
"Salam" means "peace".
"Assalamu Aalaykom" means "Peace (of Allah be) upon you". to reply we say "Wa Alaykom assalam" (reversed words) which means : "and (peace) upon you".
"Assalamu Aalaykom wa rahmatullah wa barakatuh" means "Peace and mercy of Allah be upon you".
I'm not qualified to answer your question in totality, but here are some general differences between Islam and Christianity, which concerns beliefs (I can't state the difference in rituals) :
- All muslims believe there is One God, Allah, no notion of Father and Son, which is not the case in Christianity.
"Say: He is Allah, the One! (1) Allah, the eternally Besought of all! (2) He begetteth not nor was begotten. (3) And there is none comparable unto Him. (4) " Surat Al-Iklas.
- Muslims believe christianity was originally a true religion from God, and Jesus (as) is a Prophet and a messenger from God.
- Muslims believe Jesus is human, born in a miraculus way.
"The Messiah, son of Mary, was no other than a messenger, messengers (the like of whom) had passed away before him. And his mother was a saintly woman. And they both used to eat (earthly) food. See how We make the revelations clear for them, and see how they are turned away! (75)" Surat Al-Ma'edah [75]
- Christians do not believe Islam is a true religion, and do not believe Muhammed was a messenger of God, unlike the belief of muslims.
- (a big part of) Christians believe Jesus is God.
- The meaning of a "Prophet" is not the same between Islam and Christianity.
- Muslims believe Islam is the original religion of all the prophets
"And verily We have sent among every nation a Messenger (proclaiming) worship Allaah alone and avoid Taaghoot (false deities)" (an-Nahl 16:36)
"The prophets are brothers and their faith is one"(Bukhaaree and Muslim)
- The main belief of muslims is summarized in this verse :
The messenger believeth in that which hath been revealed unto him from his Lord and (so do) believers. Each one believeth in Allah and His angels and His scriptures and His messengers - We make no distinction between any of His messengers - and they say: We hear, and we obey. (Grant us) Thy forgiveness, our Lord. Unto Thee is the journeying. (285) (Surat Al-Baqra [285])