Added FAQ#5 for this also 
With OH2, you will use by default PaperUI to setup and configure your Home Automation environment.
Optionally (for more advanced use cases) you can skip PaperUI for the creation of Things and Items and use flat config text files.
Things can be defined in $conf/things/*.things files
Items can be defined in $conf/items/*.items files
(not $conf/things/*.items)
Read more on configuration options here: Configuration | openHAB