Shopper Marketing is a fairly new discipline and there are many different definitions out there.
One definition could be:
"Shopper Marketing involves all activities by manufacturers and retailers that seek to influence the consumer's path to purchase, whether conducted within the store, online, through mobile, or outside of the physical store".
The main focus is probably the shift from consumer insights towards shopper insights. There is one area within marketing where a great deal of academic and practical research has been done; this is the area of consumer behavior. Many years of extremely rigorous research from every angle using every method available has revealed useful insights into all aspects of consumers. A growing part of this research has focused on shopper behavior. For example, Paco Underhill's work Why we buy is one of many books well known for its detailed, anthropologically driven observation work in stores. Herb Sorensen has also written a very interesting book about shopper behavior called In the mind of the shopper.
Our recommendation is to read these books and try to find the definition that suits your organisation. And don't forget that shopper insights do not need to be complicated, but they need to be business-based if your organisation is to become more shopper-centric and shopper-focused.