Short Answer:
The price, their manufacture, what they are used for and how they work.

Long Answer:
Valtrex, made by GlaxoSmithKline usually comes in a 500 mg pill and is for the treatment of Genital Herpes, shingles, cold sores.

Acyclovir, made by Various Manufacturers, usually comes in a 200mg or 400mg pill and is for the treatment of Genital herpes, shingles, cold sores.

Famvir, made by Novartis, usually comes in a 500 mg pill and is for the treatment of Genital Herpes, shingles, cold sores.

And

Denavir, made by Novartis, comes in a topical cream and is for the treatment of cold sores.