The Zombie is the resulting creature when a human is infected with certain types of engineered virus, most commonly the t-Virus
which was developed by Umbrella and based off the Progenitor virus. The virus itself is designed to increase strength and resilience although it has a devastating side effect of causing catastrophic brain damage and causing severe skin and muscle necrosis. It also does not just affect humans - it has been shown to infect any other living creature whether it be plant based or animal, for example the Zombie Dog
or Plant 42
A common misconception with Zombies is that they are the undead - this is not true. They are humans which received an infection whilst alive and as time has progressed, the virus has taken full effect and the transition into a Zombie is complete. The infection can be transmitted in various ways depending on which strain of the t-Virus
it is, although most are by a bite and large scale outbreaks have been caused with a pathogen mixed into the air or the water supply.
The Zombie itself becomes a shadow of it's former self with none of it's human personality or compassion - it is strictly an animalistic creature which only has two basic needs: feeding (most commonly a cannibalistic need to feed on flesh) and spreading the infection. Although they are slow and lack intelligence to use weapons, their resistance to firepower makes them extremely dangerous - for example most Zombies require approximately 10x 9mm rounds to the head in order to kill one. As their outer layer is necrotic, it's extremely easy to dismember a Zombie but their lack of pain receptors allows them to continue their assault without a single concern.
Umbrella have continually researched into the Zombie and therefore have produced most likely hundreds if not thousands of variations each with distinct advantages and disadvantages - although largely it has been considered more of a by-product of research than a profitable B.O.W.
Occasionally Zombies have also been observed going through mutations once they have been incapacitated or have been active for some time. Some are relatively minor changes, but others are complete evolutions - for example the Crimson Head
or the formidable Licker