What is the difference between triacs and thyristors?

They are essentially the same in terms of resistive loads. For inductive loads, however, back-to-back thyristors are effective.
Thyristors connected back-to-back or a triac can be used for Solid-state Relay switching elements.

Thyristors and the triacs differ in their characteristics to enable the elements to adjust to a rapid rise or fall.

These characteristics are expressed as dv/dt (unit: V/ μs). The dv/dt value for thyristors is greater than the value for triacs.

Triacs help downsize Solid-state Relays because they can sufficiently perform switching for motors of up to the 3.7-kW class with inductive loads, and one triac element provides functionality equivalent to two thyristors connected back-to-back.

Note: dv/dt is the rate of voltage rise.

Resistive load		Inductive load
40 A max.	Over 40 A	3.7 kW max.	Over 3.7 kW
Triac	OK	OK	OK	Not as good
Two thyristors	OK	OK	OK	OK