Cash Flow by definition is the total amount of money being transferred into and out of a business, especially as affecting liquidity. In real estate investing, what this means is:
Total Income - Total Expenses = Cash Flow
While you would assume total income would consist of just rent, make sure to include other potential sources of income including application fees, late fees, and laundry income. If these sources are possible, also make sure to estimate your numbers using a conservative approach. In the long run, this will be the most beneficial approach. On the flip side, your total expenses are NOT simply your mortgage, property taxes and insurance. Other expenses that cannot be overstated include utilities, potential flood insurance, repairs, vacancy, property management, and capital expenditures. The last three expenses can be used as percentages against your monthly income from the property. Failure to include ALL possible expenses could lead to you purchasing a "deal" that actually turns out to be no deal at all.
Once you have purchased a property and become a landlord, it is to stay up to date with the value of your property and identify whether appreciation or depreciation has taken place. While this is very important post-purchase, factoring in appreciation for an investment decision is speculative in nature and brings unneeded risk into the situation. In the event that your property has depreciated over time, there may be significant tax advantages to this and those same advantages may even be available to you if your property has appreciated over time.
Net Operating Income
Net Operating Income by definition equals all revenue from the property minus all reasonably necessary operating expenses. To look at this simply, NOI is calculated on a monthly basis using monthly income and expense data, therefore it can be converted to annual data just by multiplying by 12. The important thing to remember with NOI is that the formula does not include debt service costs, (loan costs) which differs from cash flow. One of the biggest reasons a landlord will want to know this number is because Net Operating Income plays a huge role in determining the value of your property. For this reason, it is in your best interest to work towards maximizing this number using different strategies to accomplish this.