From a chemical standpoint, Zyrtec is a combination of both levocetirizine and dextrocetirizine isomers.
It is pretty common for drugs to contain both levo and dextro isomers.
With advanced filtering (increased cost), you can isolate active isomer.
Levocetirizine (Xyzal) is the active portion.
Many times when a molecule is formed, certain chemical bonds will twist.
If the bond twists to the left you get levo-.
If it twists to the right, you get dextro-.
The 2 different molecules will have the same chemical makeup, but the activity will be drastically different.
Also note: It is not always the levo portion that is the active portion.
The isomers are sometimes labeled S (Latin sinestro meaning left) and R for right.
FYI: 10mg of Zyrtec has the same activity of 5mg of Xyzal.
Unless you have some allergic reaction or adverse effect from the non-active portion, it is just as effective (but less expensive) to use the combination drug.