The distillation process can happen either in batch or continuous stills. Batch stills are what we typically think of when we think of moonshine or whiskey being made in a pot still.
This is a smaller batch process where the pomace is placed in a pot with water and then heated until it boils. The vapor that is produced is then collected and condensed back into a liquid. This liquid is what we know as grappa.
The second method for making grappa is through continuous stills. This is a much larger operation and is more common for industrial-scale grappa production.
The pomace is placed in a large tank with water and then heated until it boils. The vapor produced is then passed through a series of condensers that cool it back into a liquid. This method can produce a lot more grappa in a shorter amount of time, but some people believe that it doesn't taste as good as batch-produced grappa.
To be fair, I don't think grappa tastes good at all, so I don't notice much difference between batch stilled and continuous stilled. Both taste like paint thinner to me. But it's tradition, so I try grappa at least once on each trip to Italy.