The Mus­lim holy month of Ra­madan start­ed last week, and most of the world's 1.6 bil­lion Mus­lims will be ob­serv­ing it. This means there's a good chance you might en­counter some­one–a friend, a co-work­er, your child's teacher–who is cel­e­brat­ing Ra­madan. But what is Ra­madan, ex­act­ly? Here are the most ba­sic an­swers to the most ba­sic ques­tions about Ra­madan.

What is Ra­madan­ac­tu­al­ly about?

Ra­madan is the most sa­cred month of the year for Mus­lims. Mus­lims be­lieve it was dur­ing this month that God re­vealed the first vers­es of the Qu'ran, Is­lam's sa­cred text, to Mo­hammed, on a night known as "The Night of Pow­er" (or Lay­lat al-Qadr in Ara­bic). Dur­ing the en­tire month of Ra­madan, Mus­lims fast every day from sun­rise to sun­set. It is meant to be a time of spir­i­tu­al dis­ci­pline–of deep con­tem­pla­tion of one's re­la­tion­ship with God, ex­tra prayer, in­creased char­i­ty and gen­eros­i­ty, and in­tense study of the Qu'ran.

At the end of Ra­madan there is a big three-day cel­e­bra­tion called Eid al-Fitr, or "the Fes­ti­val of the Break­ing of the Fast." Every­one comes to­geth­er for big meals with fam­i­ly and friends, ex­changes presents, and gen­er­al­ly has a good time.

How does­fast­ing work?

Fast­ing dur­ing Ra­madan is one of the five pil­lars–or du­ties–of Is­lam, along with the tes­ti­mo­ny of faith, prayer, char­i­ta­ble giv­ing, and mak­ing pil­grim­age to Mec­ca. All Mus­lims are re­quired to take part every year, though there are spe­cial dis­pen­sa­tions for those who are ill, preg­nant or nurs­ing, men­stru­at­ing, or trav­el­ing, and for young chil­dren and the el­der­ly.

The prac­tice of fast­ing serves sev­er­al spir­i­tu­al and so­cial pur­pos­es: to re­mind you of your hu­man frailty and your de­pen­dence on God for sus­te­nance, to show you what it feels like to be hun­gry and thirsty so you feel com­pas­sion for (and a du­ty to help) the poor and needy, and to re­duce the dis­trac­tions in life so you can more clear­ly fo­cus on your re­la­tion­ship with God.

Dur­ing Ra­madan, Mus­lims ab­stain from eat­ing any food, drink­ing any liq­uids, smok­ing cig­a­rettes, and en­gag­ing in any sex­u­al ac­tiv­i­ty, from sun­rise to sun­set.Do­ing any of those things "in­val­i­dates" your fast for the day, and you just start over the next day.

Mus­lims are al­so sup­posed to try to curb neg­a­tive thoughts and emo­tions like jeal­ousy and anger, and even less­er things like swear­ing, com­plain­ing, and gos­sip­ing, dur­ing the month.