If you have extra time with you over weekends or after your job, you can freelance and take small projects. There are many websites like elance.com , odesk.com or freelancer.com where you can earn money by working on projects. There are thousands of people in India who are working full time on these kind of freelancing websites and believe me, they are earning in lakhs. Look at it as a serious way of making money and not just a time pass activity.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.