Simplified Definition: It is a private subsection of AWS that you control, into which you can place AWS resources (such as EC2 instances and databases). You have full control over who has access to the AWS resources that you place inside your VPC.
AWS Definition: "Amazon Virtual Private Cloud (Amazon VPC) lets you provision a logically isolated section of the Amazon Web Services (AWS) cloud where you can launch AWS resources in a virtual network that you define. You have complete control over your virtual networking environment, including selection of your own IP address range, creation of subnets and configuration of route tables and network gateways."
Note: When you create an AWS account, a "default" VPC is created for you.