Summarize this article in one sentence.
If you want to greet someone in Hebrew, "shalom" (shah-lohm) is the main word to use. It's appropriate as a greeting regardless of the context, the age of the person you're greeting, or how well you know them. On the Sabbath (Saturday) you can also say "Shabbat Shalom" (shah-baht shah-lohm), which literally means "Sabbath peace" or "peaceful Sabbath." This greeting is commonly used in Israel. Like "shalom" by itself, it's appropriate in any situation when greeting anyone. This greeting is related to the Arabic greeting "salaam alaikum" and both greetings mean literally the same thing: "peace upon you." There's a lot of overlap between Arabic and Hebrew because the two languages belong to the same language family. "Ahlan" is borrowed from Arabic. Hebrew speakers use it in the same way Arabic speakers do, as a simple "hi." While it's far more casual than "shalom," you can still use it to greet anyone, young or old, in casual contexts. In a more formal situation, or when speaking to someone in a position of authority, this may be too casual a greeting.
Say "shalom" in any situation. Switch up your greeting by saying "shalom aleikhem" (shah-lohm ah-ley-khem). Use "ahlan" (ah-hah-lahn) to say "hi" more casually.