Is there a thin line between secondary income (tax) and extra 'hobby' income (no tax)?

Sorry if it sounds like a stupid question lol ... but I know someone who works but also has their own business from home (clothing alterations - if you're jeans are too long etc) and she has to file for secondary tax (her and husband); now what if you are making jewelery or selling other craft or paintings or whatever online (e bay) or at the market which you dont have to file for tax?? Esentially they are both doing the same thing are they not, getting more money for their services? Someone please tell me what's the difference? Seems the same to me lol :/