Default What ART does for regular Joe?

Hi!

Is there simple explanation what ART does for regular Joe? Why does Google gives me choice between ART and Dalvik?

Thanks!

Sent from my Nexus 4 using xda app-developers app
Nexus 4 - stock, rooted
LG Optimus 3D - P-Vlad ROM - for wife
Samsung Galaxy S - Slimbean - as iPod for daughter
Acer Iconia A500 - stock - for daughter
Nook Color - CM7 - for son