Google Analytics shows to directly measure traffic of all visitors to your website, Alexa uses modeled data based on web use of those with the Alexa Toolbar installed.

We see that the difference in Alexa vs Google Analytics is because of the way that the data is collected.  As we know that, Google shows of the data that it collects from the actual websites.  Alexa counts visitor information from web surfers with the Alexa toolbar installed, which shows a very small part of the total web traffic.

The number of visitor calculation from Google Analytics, comes from the execution of on your webpages, each time a visitor load one of your pages. Search engine catches, but overall estimations are pretty accurate.

The number of visitor calculated by Alexa recorded by people that have installed Alexa toolbar in their browser. Since the amount of people browsing with the Alexa toolbar is very small. These visitor calculation are largely inaccurate.

Google analytics displays visitor's activities, real time user,  backlinks, out bound links, time spent on pages by visitors and in-deep insights webpage performance and moves required to create your site user friendly and Search Engine friendly....so as to give a accurate results.

Alexa is not important tool now. It displays few data to compare domains but it not a very good source to get data about traffic.

Alexa differs technically from Google Analytics because Google Analytics gives much more detailed information about the website and some comparative information via it’s benchmarking feature.

As we know that, Google has all of the information that it collects from the actual websites. Alexa take data from web surfers with the Alexa toolbar installed, which shows a very small area of the total web surfing traffic.If we have better Alexa rank then it is not necessary that website having great traffic. Lots of example examples for that. 

To conclude: We should focus on Google Analytics data.