NetScaler is a hardware device (or network appliance) manufactured by Citrix, which primary role is to provide Level 4 Load Balancing. It also supports Firewall, proxy and VPN functions

it is both Physical and Virtual Device that enable the SSL offload, Encryption, compression of outgoing and incoming traffic.

Different types of Net scalar Devices

VPX: its is Virtual Image of the net scalar binaries which can be hosted on Xenserver, VMware, KVM’s

MPX : it is Physical Hardware that is manufactured by Citrix in Netscalar ADC has been Integrated with Hardware as bare metal Device.

SDX : It is Combination of Both Physical and Virtual appliance.In simple Terms it has XenServer Kernel which host multiple VPX on Hardware and enables the Best performance for huge load

you can compare the Edition below

http://www.citrix.com/products/netscaler-application-delivery-controller/buy/editions.html

you can compare Netscalar Devices Below

http://www.citrix.com/content/dam/citrix/en_us/documents/products-solutions/netscaler-data-sheet.pdf