it means May Peace Be With You. Muslims use it everytime they greet eachother, in place of hi or hello - which dont really have a meaning in themselves
no. the longer one give you more blessings.
you might find some people say assalamuaaikum warahmatullahi barakatu - again this will give more blessings upon the person who says it. also when meeting a musilm the person who gives the salaam first wll get more belssings
Salaam means peace, and Salaam 'alykum - may God's peace be upon you. (so its kind of like a supplication for the person you meet.) Which is better than just sayin 'hi' or 'hello' which dont really mean much.