Sulfur and chlorine are both on the "right side" of the periodic chart. Therefore they are both nonmetals. Nonmetals will react with each other and form covalent bonds. Sulfur and chlorine are also Representative Elements which means when they react they will generally follow the octet rule.
Representative nonmetals, forming covalent bonds, will share eight valence electrons each.
Also, we must remember that chlorine is a diatomic molecule which means when we say chlorine we really mean the chlorine molecule Cl2. That is, when it reacts it provides two atoms of chlorine for the reaction.
Sulfur is in Group VIA (or 16) which means it has six valence electrons to start. Four of those electrons are paired together and the remaining two are on their own.
Chlorine is in Group VIIA (17) so it has seven valence electrons. Six of those electrons are paired together, and one is on its own.
When chlorine reacts with sulfur one of the unpaired electrons in sulfur shares the unpaired electron in one of the chlorines. The other unpaired electron in sulfur shares the unpaired electron in the other chlorine. So you get
S + Cl2 ---> SCl2
Which is read as "one sulfur atom reacts with one chlorine molecule to form one sulfur dichloride"