Primary Memory is the memory that is directly accessible to the CPU. The computer needs this memory for constant processing of data and instructions. It reads instructions and executes them. RAM is a kind of Primary Memory. It is small in size, light but expensive also.
Whereas a Cache Memory is a kind of memory which is used by the Central processing unit of a computer. It reduces the time to access the main memory. It is smaller in size, faster and stores copies of data from frequently visited locations.
Primary memory is connected to the CPU Through a memory bus or a front end bus. RAM is used as primary storage having all such storage such that does not need to start the CPU.