Arun Sirpal compares the Target Server Memory and Total Memory Perfmon counters:
Target Server Memory (KB) is the amount of memory that SQL Server is willing (potential) to allocate to the buffer pool under its current load. Total Server Memory (KB) is what SQL currently has allocated.
Using SQL Server 2014 developer edition (64 bit) my machine has 12GB RAM and maximum server memory is currently set to 8GB and for the purpose of this post I have set minimum server memory to 1GB (Lock Pages in Memory has not been set).
Read on for a nice description laden with Perfmon pictures.