If you are thinking about purchasing property to then rent it out, make sure the monthly rent you charge is more than your monthly mortgage loan payment. This will create a positive cash flow from your investment. This can be done with more than just real estate. You can buy a car and rent it out and you could by a yacht and rent it also. Shares can also be bought and rented and that is a strange idea, although it is different that property.
To understand this, you must first understand options. The risk involved is similar in some ways and different in others. Share prices can go up and down and they can stay where they are, but if it drops, money would be lost with rented shares. Share prices are generally more volatile than those of real estate but seeing the loss or gain happens faster with shares. If the option buyer chose to use that option, you would be forced to sell your shares.
These share can also turn into worthless shares which cannot happen with real estate.