Muslims and even those who aren't Muslim but live in the Middle East or have some Arab culture in them greet each other with "Assalamu Alaikum," or "Salam" for short. Even those who don't use this greeting have heard it, or know of it. What does Assalamu Alaikum or Salam in its essence, mean?
Literally, Salam means "peace." I have a theory that the greeting "peace" at the end of a conversation came from "salam." The greetings "peace" and "peace out" started being used in the 1960s, when cultural references were being used and referred to in hip hop music etc. THis is just a theory.
Anyway, "Assalamu Alaikum" means "Peace be upon you."
Salam = peace
3alai = on/upon
kum = you
The proper response is "Walai kum as-salam" - This means literally "and upon you be peace"
wa = and
3alai = on/upon
kum = you
as-salam = peace
What's this mean?
When someone starts off a conversation by saying "peace be upon you" or "peace be upon you too," that implies that "I will do or say nothing to harm you." It carries a weight with it, like saying: I genuinely wish peace for you, and I will do my part to keep my attitude towards you peaceful. And not just on the surface, but in the heart too.
To start off a conversation with "Peace be upon you," and then say something rude, act disrespectfully, act with vain, or say something that is insincere, is purely contradictory. Or to be negative in any way after saying "peace be upon you" is ironic and counter-productive.
ORIGINS of this greeting:
Abu Hurairah narrated that the Prophet Muhammed (pbuh) said: "Allah created Adam from dust after He mixed the clay and left him for some time until it became sticky mud, after which Allah shaped him. After that Allah left him till it became like potter's clay [a sounding clay]. Iblis used to go past him saying 'You have been created for a great purpose.' After that Allah breathed His spirit into him. The first thing into which the spirit passed was his eye and then his nose. He [Adam] sneezed. Allah said: "May your Lord have mercy upon you, O Adam! Go to those angels and see what they would say.' So Adam went and greeted them. they replied saying: "Peace be upon you and the mercy and blessings of Allah." Allah said: "O Adam! This is your greeting and that of your offspring." (Sahih Bukhari).
لَا يَسْمَعُونَ فِيهَا لَغْوًا وَلَا تَأْثِيمًا
There (heaven), they here no vain speaking nor recrimination. (56. Surah Al-Waqaih : Ayah 25)
إِلَّا قِيلًا سَلَامًا سَلَامًا
Except they say salaam, salaam. (56. Surah Al-Waqaih : Ayah 26)
These verses are recited here at 1:42