sample size, sampling error

could someone please explain the concept of sampling error? i can't really find it in my text book

ahh thanks :)

also i am stuck on two more problems. (last ones, i swear! haha)

st dev=1200

want a 95% confidence interval for the population value of annual telephone expenditure with an error of estimation (sampling error) no greater than $50

what sample size should be used?

suppose that we want a 95% confidence interval for the population value of annual telephone expenditure that is no wider than $100

what sample size should be used?