As the others have already mentioned, Zillow, Trulia, eppraisal and other sites use their own custom mathematical algorithms to determine price values based on listing, pending and sold prices of surrounding homes within specific geographic locations. When you consider the enormity of this job and the sheer number of homes sold in the United States, any website that can generate estimates that are even "close" to the actual price--well, that's pretty amazing.
Unfortunately, some of the most important factors in "pricing" a home--school district, configuration, location, interior condition--are not considered in these mathematical equations, so much so that even Zillow admitted recently their estimates can be as much as 20 percent off the price for a home.
Truth is, the only way for you to really determine price is to pay for an appraisal to obtain a value.
Otherwise, as mentioned in the past by many of us here on Trulia, the amount of money a home is worth is based solely on what the buyer is willing to pay and the seller is mutually willing to accept. Most importantly, Hironmoy, your best answers and those tailored to your specific needs and to any home to which you are considering will come from your own agent--hopefully, someone who is local in the area and understands the intricacies of pricing homes here.
Grace Morioka, SRES
Area Pro Realty