Real Estate investing is when you purchase a real estate property that generates income or is otherwise intended for investment purposes rather than as a primary residence. These properties are used to generate Cash flow through rental income and profits and to build equity through appreciation.

Equity is the value of a piece of property over and above any mortgage or other liabilities relating to it.

Example:  Let's say you get a great deal on a property for $100K and your initial investment is $20K.  At this point your Equity is $80K. 

You then make improventes for $10K over the course of 1 year and at the same time paying off the principal.  After 1 year you bring the value of that property to $130K.  Your equity on the property is now more than $100K.

Cash flow is the total amount of cash that is generated after all expenses are deducted from all income received.