KB,
I'm a numbers person; I enjoyed David Collins' explanation (and Will's additional commentary on David's comment). If you're like me, then perhaps seeing an example of how to work the numbers will help to add more teeth to all of the other great information already provided in this post.
ptcf = NOI-DS = DS(DCR-1) = dp(ROI)
cap rate = NOI/pp
(where ptcf is the pre-tax cash-flow, DS is the debt-service [or 12 months of mortgage payments], DCR is the debt-coverage ratio [or NOI/DS], dp is the down-payment, NOI is the net operating income, ROI is the return on investment, and pp is the purchase price)
NOTE: I'm going to make a few statements using the form A is B, and I'm using is in the mathematical sense--but not equating A and B as terms.
First, if one were to purchase a property with all cash, then the following are true: 1) the purchase price is the down-payment, 2) the ptcf is the NOI, and 3) the ROI is the cap rate. For example, let's say one purchases a property with the following characteristics: pp=120K, NOI=12K, and DS=0 (due to an all cash purchase).
cap rate = 12K/120K = .1
ptcf = 12K-0 = 12K
ROI = ptcf/dp = 12K/120K = .1
Second, if one were to purchase a property correctly with positive leverage, then ROI is greater than or equal to the cap rate. For example, let's say one purchases a property with the following characteristics: pp=120K, NOI=12K, DS=10K, and dp=12K (which implies a loan at 90% LTV).
cap rate = 12K/120K = .1
DCR = 12K/10K = 1.2
ptcf = 12K-10K = 2K
ROI = ptcf/dp = 2K/12K = .167
Notice how the purchase price, NOI, and cap rate are the same in both examples; yet, the ROI differs. I'll show three more leveraged examples to help drive the points home.
Following is an example of positive leverage misapplied (ie the ROI is less than the cap rate). Let's say one purchases a property with the following characteristics: pp=120K, NOI=12K, DS=10K, and dp=24K (which implies a loan at 80% LTV).
cap rate = 12K/120K = .1
DCR = 12K/10K = 1.2
ptcf = 12K-10K = 2K
ROI = ptcf/dp = 2K/24K = .083
Let's say one purchases a property with the following characteristics: pp=120K, NOI=12K, DS=9.6K, and dp=24K (which implies a loan at 80% LTV).
cap rate = 12K/120K = .1
DCR = 12K/9.6K = 1.25
ptcf = 12K-9.6K = 2.4K
ROI = ptcf/dp = 2.4K/24K = .1
Let's say one purchases a property with the following characteristics: pp=100K, NOI=12K, DS=8K, and dp=20K (which implies a loan at 80% LTV).
cap rate = 12K/100K = .12
DCR = 12K/8K = 1.5
ptcf = 12K-8K = 4K
ROI = ptcf/dp = 4K/20K = .2
By the way, it's OK if you don't understand how all of these numbers work together right now. You can learn that over time. For now, it's important for you to understand that there really is a method to the madness, and that there is a way to use real numbers to quantify how leverage can work for/against you. And it's ultimately up to you to structure your deals henceforth, so that the numbers will work for you--whether or not you opt to use leverage.