What are the typical three different categories of VPNs? 

OSI layer 1 defines the standards for cabling and connectors.

Different Memories used in a CISCO router are as follows :

NVRAM stores the startup configuration file.

DRAM stores the configuration file that is being executed.

Flash Memory – stores the Cisco IOS.

By default, all Windows 2000 trusts within a domain tree or forest are transitive trusts.

PPPoE  or  PPPoA

A VPN allows or describes the creation of private networks across the Internet, enabling privacy and tunneling of non-TCP/IP protocols. A VPN can be set up across any type of link.

IPSec is an industry-wide standard suite of protocols and algorithms that allows for secure data transmission over an IP-based network that functions at the layer3 Network layer of the OSI model.

If you have a serial port configured with multiple DLCIs connected to multiple remote sites, split horizon rules stop route updates received on an interface from being sent out the same interface. By creating subinterfaces for each PVC, you can avoid the split horizon issues when using Frame Relay.

ATM is the Data-Link layer protocol that's typically used over the DSL layer1 connection from the CPE and terminated at what's known as the DSLAM - an ATM switch that contains DSL interface cards, or ATU-Cs.