Google Analytics segments, formerly called advanced segments, are among the many useful yet underutilized features in Google Analytics.
There are two basic kinds of segments: pre-built and custom. The real power of segments lies in those you define yourself as custom segments, but many of the pre-built segments are also very handy.
For example, you can apply the pre-built Mobile segment to temporarily restrict your reporting to visitors who have accesses your site on a device that Google Analytics has recorded as the mobile Device Category dimension, that is, smartphones and devices such as Apple iPods with smartphone-sized viewports.
If you wanted to see specifically Android smartphone user interacted with your site, you could easily define a custom segment as Device Category exactly matches mobile and Operating System exactly matches Android.
Make sure to take full advantage of the segments feature in Google Analytics. Segments are flexible and retroactive, they affect only your display without permanently altering the underlying view data, and they’ll provide insights more quickly than the default All Sessions report display.