Allegra-D is a second generation antihistamine with a decongestant added on (that's what the "d" stands for). Second-generation just means it's one of the more modern antihistamines that have less side effects, especially drowsiness than the old standbys like chlor-tri-meton and Benadryl. Most people who have allergies will get adequate relief from a plain antihistamine, but if nasal stuffiness is a big problem for you, then a decongestant added to the antihistamine can offer another level of relief.
Allegra, Claritin and Zyrtec are all second-generation antihistamines that work very similarly, as is the newer Clarinex. They each come in the D varieties, with the added decongestant. Allegra is only available by prescription, while Claritin and Zyrtec can be bought over the counter. I think you may need a prescription for Claritin-D and Zyrtec-D, though. At the very least, they're probably kept behind the pharmacy counter as very few decongestant medicines are now put out on the shelf, even if they don't require a prescription.
At any rate, I always recommend you discuss medication changes with your physician first. Your pharmacist is another great source of information and advice.
We hope you find this general health information helpful. Please note however, that this Q&A is meant to support not replace the professional medical advice you receive from your doctor. No information in the Answers above is intended to diagnose or treat any condition. The views expressed in the Answers above belong to the individuals who posted them and do not necessarily reflect the views of Remedy Health Media. Remedy Health Media does not review or edit content posted by our community members, but reserves the right to remove any material it deems inappropriate.