CheckStyle is a development tool which helps you format your Java code with respect to industry accepted standards. You can create your own check list to format and check style of code, or simply use one provided by checkstyle.
For eclipse it is easy to install plugin that is available. Go help->Eclipse MarketPlace-> serach for checkstyle.
Once eclipse plugin is added, you will see option on right click of project or a file to run a checkstyle which will show error messages in panel (Window->Show View-> Others-> Search Checkstyle).
If you want eclipse to format (Ctrl+Shift+F) the code in desired format, you can add formatter XML by Project->Properties->Formatter->Import. One such default XML can be found at – https://github.com/sevntu-checkstyle/sevntu.checkstyle/blob/master/sevntu-checks/CheckstyleFormatterForEclipse.xml