Maybe Jack can tell us: what level of activity makes one a "professional"?
This will vary (widely) by company and stae, hence the following is only a guide. As always, check with your own provider to be certain of how your coverages will apply.
The word often used to distinguish between pro and not is "incidental..." Meaning, is the amount of income derived from the activity "incidental" to one's primary livelyhood, or does it represent a more significant portion? The obvious problem with the above it is vague...
Bottom line is if you're a pro, get the real business/pro coverages. If your an amatuer, you probably don't need anything more than the vanilla personal lines coverages.
If you fall inbetween, I recommend the following regardless of how much you earn doing whatever it is you do:
1) If you have a website listing services or products with pricing, get the pro policy(s).
2) If you only participate in the annual church art sale, the normal policy(s) are probably just fine.
3) If you participate in the annual church art sale, the local Rotary art sale and lead a few friends on a photo workshop for a long weekend each year, you're pushing the line and probably need the pro policy(s).
Hope this helps,