CAP rate = Capitalization Rate. Basically, if you paid all cash for a property it is the return you would receive on your money based on the income the property produces. So, if you pay $1MM for the property and it throws off $100k a year, it would have a CAP Rate of 10%...or it would be a "10 CAP" if you are in the know.

NOI = Net Operating Income. This is the income after you have deducted vacancy losses and normal operating expenses (but, not debt service aka: loan payments). Usually, you can figure what vacancy is running for a particular property type (B apartments, office space, retail, etc) in a particular area. You take that factor (say 2%) and subtract it from the total income the property produces. Then you will subtract out your normal expenses (ie: utilities, management, repairs, advertising, etc). What you have left is your NOI. This is before your debt service or loan payments.

CAP rates are not a good way to evaluate a piece of income property as they can be misleading. Many times they are based on what we would call Pro-Forma (or expected) numbers rather than actual numbers. There are really many things that can make this a bad indicator of an investment's likely performance. But, CAP rates are a good way to quickly qualify a property along with a few other numbers.

NOI is just another piece of the puzzle as well. Neither affect the property one way or another. They are just terms brokers and investors use to describe how a property performs.