vSphere supports two models of storage provisioning, thick provisioning and thin provisioning.

Thick provisioning
It is a traditional model of storage provisioning. With thick provisioning, large amount of storage space is provided in advance in anticipation of future storage needs. However, the space might remain unused causing underutilization of storage capacity.
Thin provisioning
This method contrast with thick provisioning and helps you eliminate storage underutilization problems by allocating storage space in a flexible on-demand manner. With ESXi, you can use two models of thin provisioning, array‐level and virtual disk‐level.