In a Double Replacement Reaction, different atoms in two different compounds replace each other. In other words, two compounds react to produce two new compounds. The generalized equation for a double replacement reaction is as follows...
AB + CD ________> AC + BD
In this reaction, the elements from both compounds switched places, or replaced each other, in the compounds to form two new compounds. A, B, C, and D represent elements on the periodic table.
NaOH + HCl ________> NaCl + H2O
in this case, the sodium and the hydrogen replaced each other and formed two new compounds from the original compounds.