In a synthesis reaction, a new product is formed from two or more reactants. For example:
A + B → AB
Decomposition is the opposite of synthesis:
AB → A + B
In a single replacement reaction, one part of the compound is replaced with something else:
AB + C → AC + B
A neutralization reaction occurs when an acid is added to a base to produce a salt and water. In general:
acid + base → salt + water
A specific and common example of neutralization is:
HCl + NaOH → NaCl + H2O
This reaction is actually a double replacement because two compounds are "switching"... the Cl and OH (hydroxide) switch places.
For the second part of the question:
Cl + KI → KCl + I
Both Cl and I are in Group 7, which means that they each have a charge of -1. K is in Group 1 and has a charge of +1. Negative 1 plus positive 1 equals 0, which is what we want.
Cl and I exist as diatomic molecules:
Cl2 and I2
So, in order to balance the equation, we must write it with coefficients so that there are the same number of elements on both sides of the equation:
Cl2 + 2 KI → 2 KCl + I2
NOTE: The numbers after the element symbols must be written as subscripts. Coefficients written before the compound or molecule tells us how many compounds or molecules are in the reaction.