An Azure virtual network (VNet) is a representation of your own network in the cloud. It is a logical isolation of the Azure cloud dedicated to your subscription. You fully control the IP address blocks, DNS settings, security policies, and route tables within this network. You can also further segment your VNet into subnets and deploy VMs. Additionally, you can connect the VNet to your on-premises network.
VNets are associated to a resource group which provides network boundaries for resources. A VNet is required when deploying a VM. VNets cannot span over multiple locations.
For information, see the Azure Documentation Center and search for Virtual Network Overview.