what is inflation?

After a long time I went to market to buy vegetable. I was shocked to find the price has increased multifold.I was under the impression as government is claiming that inflation rate is in a negative zone,the daily essential commodities which we common people need have become cheaper.So I am puzzled on what basis our finance department calculates the price index.We common people will be bluffed by any government which comes to power for decades and decades.