Difference in a Matte and a Mask

[Todd Kopriva]

A mask is a path that is used to define an area of transparency. Very crudely speaking, you draw a mask and it essentially says “Anything outside this shape is transparent; anything inside is opaque.”

A matte is a layer whose values (for example, brightness values) are used by a different layer to define areas of transparency.

There’s more explanation, including pictures and links to some lessons by Aharon Rabinowitz here on the COW, in the “Alpha channels, masks, and mattes” section of After Effects Help.

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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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[Stuart Elith]

Hey, we all have to learn somehow!

Just a few little thoughts from my experiences (mostly in compositing/VFX, but a bit of motion graphics).

I tend to use masks most of the time, for a few reasons : Firstly, they are quicker to apply than a track matte, so for simple stuff, like if you only want part of an image to show up, you can just grab your pen tool and draw away, and you’re done.
Applying a track matte takes an extra layer (which also creates clutter) and while they’re still pretty straightforward, i find it less convenient.

Basically I mainly seem to use track mattes for circumstances where I need to get a more complicated alpha – for example, using a fractal noise layer as a track matte means that the underlying layer will have detailed, varying levels of opacity… good for interesting fades/transitions, or stuff like reflections in water, where you only want the reflection to show up in the brighter parts of the water.
It’s thinking about alpha in a different way than masks.