1.EC2 is a aws service that is used to create virtual servers or instance in aws console.
2. Security group is a firewall for the instance.
3.Private key is downloaded locally on laptop or pc. It's role is to decrypt password. The public key is stored directly on the instance.
4.It is rdp(remote desktop protocol). It is used to connect windows desktop or instance remotely.
5. Instance is a virtual server or machine .