Basics

Metal That Has Been Extruded into a rectangular prism shape with at least one side having a t shape indentation
Allows for easy attachment of componets via sliding a specialized nut into the slot, which becomes a Captive Nut of sorts
Essentially it is like Gridbeam but with an infinite amount of potential "holes" between each end of the beam
High quality t-slot materials also tend to be level and square etc, and thus are often used in cnc machines

Sourcing