Side work is specific work that you are performing while employed in that same field but not for your employer.
I have my day job, which not an HVAC business, I'm a manager. I have a second job, which is run my HVAC business. I have a business model and customer that allows me to focus on my customers after hours and on the weekends. Although it is a second line of work it is not side work. I take it very seriously and run it as a business.
I am licensed, insured, report all income, and treat it as a business, not a hobby. A hobby is something you do to occupy your time. If you want a hobby go race R/C cars and leave the serious work to the serious contractors. Put your foot through someones ceiling or have your buddy fall off a ladder while changing out someone's system and you'll realize real quick what the difference is between "side work to help make ends meet" and "side work that cost me my house and home because I wasn't insured and bonded".
Irishfan's mistake was submitting a question (and a pretty imposing one, at that) to a varied audience and not considering the implications. I'm going to assume (not ignoring the risk) that he's never operated a business, made payroll, stared at a balance sheet or worked 18 hours a day weeks on end to make sure all of his employees and customers are taken care of.