Azure Virtual Network - Highlights

Azure resources communicate privately, directly, and securely with each other over virtual networks.

VNet is scoped to a subscription. You can implement multiple virtual networks within each Azure subscription and Azure region.

VNet is scoped to a single region/location; however, multiple virtual networks from different regions can be connected together using Virtual Network Peering.

from Learn Azure in a Month of Lunches

Your subnets should not cover the entire address space of the VNet. Plan ahead and reserve some address space for the future. If you create the smallest available subnet of /29 (with eight IP addresses), Azure will retain five addresses

It is recommended you have fewer large VNets rather than multiple small VNets. This will prevent management overhead.

Each virtual network is isolated from other virtual networks within each Azure subscription.

Vnet does NOT span Regions, it can span availability zones.

You can connect one VNet to another VNet using either Virtual Network Peering or Azure VPN Gateway

Transitive peering is not supported i.e. if you peer VNetA to VNetB and VNetB to VNetC, VNetA and VNetC are not peered.

Not all Azure services reside in the customer's virtual network. The majority of Azure data services such as Azure Storage, Azure SQL, and Azure Cosmos DB, are multi-tenant services that can be accessed over public IP addresses. 

Key links - 

Azure Virtual Network frequently asked questions (FAQ)