To clarify what Perry is saying: A foreclosure is not the exact same as a short sale. Well, it is, but really, a short sale is, in essence, a property in pre-foreclosure -- part of the foreclosure process.
The difference is that it is still in the owner's possession, but requires bank approval to be sold (for an amount less that what is owed to the lender). A foreclosed property has already been sold at public auction. This happens should a short sale become unsuccessful, or not agreed-to by the lender.
Sometimes, the outstanding balance owed is high enough that nobody will be willing to offer the minimum bid and the property is not sold at a public auction. At this point it becomes an REO - Real Estate Owned property - and goes back to the lender.
Anyway, in answer to your actual question: In general, making an offer on a short sale is sort of like making a regular offer - at first. The offer has to be approved by the seller, THEN has to go the bank and be approved by them, as well.
Typically, a short sale offer must be accompanied by "proof of funds", if cash, a pre-approval or commitment letter from your lender, and be prepared to pay earnest money with certified funds (this would be the same with a foreclosed property, as well).
Also, be prepared to buy the property in "as is" condition. You will likely be able to obtain an inspection, but for informational purposes only. The seller will not usually make repairs.
Then, be prepared to wait. This process can drag on forEVER.
You also want to be extra sure you do your homework prior to making an offer, i.e., know the situation on any back-taxes and/or assessments. In "short", you don't want any surprises should your offer be accepted.
Be sure you are working with an Realtor or attorney experienced in this process. If you are going at it alone, you will need to be familiar with all of the forms of the short sale process, and in dealing with the bank and its process.