This is a common question we get asked as it can feel confusing, the below snippet was taking from a website explaining the difference.
Leading-edge dimmers (TRIAC dimmers)
Leading-edge dimmer switches are cheaper and simpler than trailing-edge, and were used originally to dim incandescent and halogen bulbs or wirewound magnetic transformers.
They use a ‘TRIAC’ (Triode for Alternating Current) switch to control power, and are sometimes called TRIAC dimmers.
Many existing leading-edge dimmer switches have a relatively high minimum load, which often rules out their use with modest LED or CFL lighting circuits.
However, leading-edge dimmers are by far the most common dimming control in existence.
Trailing-edge Dimmers (Reverse phase dimmers)
Trailing-edge dimmers are more sophisticated than leading-edge dimmers.
They provide a much smoother dimming control, absent of any buzzing noise, and is ideal for use in most premises.
A trailing-edge dimmer has a lower minimum load than leading-edge dimmers, making it a better choice for dimming modestly sized low-powered lighting circuits.