The property tax figure cited in the ads is normally what was paid in the prior year. Property tax is a tax based on the "current market value" of the property - and usually assessed only on land and buildings (although in some states there is a general "property tax" that include the value of your car and other large items, like boats, campers, etc.).

Many mortgage lenders will insist that you make monthly payments toward the property taxes along with the mortgage payment. Normally, the property taxes fall due all at once in the middle of the year, but the bank will set up a reserve (and usually pays a nominal interest on this balance) and often will settle the actual tax bill for you.

Property tax is a tough one right now, as property values are falling, and so towns are losing revenue - so rates tend to go up to allow the towns to continue to provide services at the same level they have been doing.

You may want to consider renting for the first year or two you are in the US until you figure out the tax system and to allow the current housing troubles to settle out. Prices are still going down, and towns are having trouble coping with how to adjust their tax systems to deal with the fallout. There are a number of studies out there that show that renting may actually the economical choice - at least until rents catch up with house prices.
Cheers,
Bev