Isolated vs non-isolated triacs
Isolation in this context is an entirely physical packaging aspect.
In say an unisolated (or normal) TRIAC in a TO22 package the metal tab will usually be the Anode which is used in conjunction with the gate for turn on or what would be the Cathode in an SCR. 

In an isolated TO22 package TRIAC the metal tab still exists but is completely encased electrically in an insulator (usually a plastic) including the inner surface of the mounting hole. The insulation is designmed to be adequate that the TRIAC can be placed on and secured to a heatsink at any potential difference liable to be encountered in a circuit where it is working within its ratings.

Data sheets will give details but I'd expect a 400V rated TRIAC to have say 1000V insulation nad probably more.

The same effect is achieved in a normal TRIAC with insulating mounting hardware. More fiddly, needs extra pats, easier to get wrong. Unisolated will usually be cheaper in material costs but may be more costly all up in a production environment. 

For amateurs isolated TRIACS used at mains voltages make staying alive and keeping your circuit alive appreciably easier.