A confidence interval is a range of values that describes the uncertainty surrounding an estimate. A confidence interval is also itself an estimate. It is made using a model of how sampling, interviewing, measuring, and modeling contribute to uncertainty about the relation between the true value of the quantity we are estimating and our estimate of that value. We can increase the expression of confidence in our estimate by widening the confidence interval.
Confidence intervals are an important reminder of the limitations of the estimates.
For a discussion on confidence intervals for the difference between two estimates, please go to General Cautions about Comparisons of Estimates .