If you wait until is listed with a realtor, then the process is pretty much the same as any purchase, since the agent(s) know how to deal with this situation. (However, expect delays and ambiguities during the process.) If you have the finances, you can buy the property at sheriff sale or even contact the owners before it actually goest to sheriff sale. But be warned, these last two options present some very unique pitfalls. And a lot of up front investigation is required for each property that peaks your interest. And often after doing all you investigation, someone else (often the bank) will bid higher than you want to go and all the time you've invested is chaulked up to additional experience.
REO's take longer to complete and rarely negotiate for any problems found during inspections.
Your local realtor can show you all of the available REO properties in your area.
There is much more to all of these transactions and I suggest you contact a real estate agent.
I have a FREE list of local and National foreclosed homes available on my website http://www.DennisStrange.com