When you have a set of data points you can adjust a line to them with a regression.

 

In a linear regression a line is adjusted to the points and the slope and interception of the line is calculated.

 

If you use Excel you may like to use also this free addon

http://regressit.com

 

 

It creates graphs of the linear regression with a 95% confidence interval.

Analysis of linear regression

The quality of the adjustment can be seen from the plot or from values of R^2.