Let's take an example of different processors(along with 
their independent caches) sharing the same memory system. 

Now if this memory is a Read-Write memory and let's say 
that one of the processors writes to this memory after some 
data computation, all the processors need to update their 
individual cache copies of the now modified memory. To do 
this the concept of snooping is implemented. cache snooping 
is the means by which each cache constantly 
monitors/detects the bus for any write to a memory location 
and if a write is found, it invalidates it current copy of 
cached memory data and copies the new content over.