SLI and crossfire are the interfaces for nVidia and ATI respectively. They allow you to link two or more Graphics cards as if they are one card. they share the Ram and processing load. The motherboard, PSU and cards have to support the interface for it to work.
The dedicated physics card is when someone uses a nvidia card that has PhysX just for the physics processor. Usually, a card handles all the games commands and code in one processor. The nVidia cards that have a PhysX core, instead seperate the code that is used for PhysX to one processor designed to handle the math of the Physics environment. You can have two cards and one of them is used just because it supports PhysX.