If you try it and keep using it you are obliged to pay what the author asks. Usually there's a ReadMe document or similar to explian the cost and how to pay. If, on the other hand, you don't like the software, you can just Trash it. It's as easy as that. There are all kinds of variations on these basic themes.
Some companies make demos of their software available.
These may be fully functioning or limited in some way. Some authors ask for a postcard, email or charitable donation rather than money. Nagware will nag you to pay. You might find some kind of time delay built in so that you can't start using the software or click a button until you've seen a reminder screen for a few seconds. Sometimes the software is limited until you pay.