Thanks pumpkin.
Lex is on the right track that "Acts" are "single goal or action", but what can get confusing is that acts can have different scopes.
For example:
The group has an ultimate goal to kill a Troll that is plauging a town. This in itself is a "goal" and can make up an Act (especially if linked to travelling to the town and leaving the town, etc)
Now this is broken down into smaller chunks
- get information from townsfolk about the monster
- search the forest for the monster's lair
- kill the monster
Each of these is a single goal/action too, and thus can be defined as an Act.
To further compound matters, each of these might be able to be broken up into their own sub-acts. For the town, say...
- Local thieves (starving) attempt to rob PCs
- Local constabulary attempts to arrest PCs
- PCs are hired/coerced into seeking and destroying the local monster
And so, these are still single goals/actions and thus acts. And so on, based upon the GM's desire for structure and depth.
An adventure structure could (theoretically) look like:
Act I
Act i
Act a
Act b
Act c
Act ii
Act d
Act e
Act f
Act iii
Act II
Act III