Difference between Cache and Tightly Coupled Memeory

You can easily find the answer in Nios/sopc documentation, which I think is more clear and detailed than what I write here.

Anyway the difference is this:

While both are very fast accessed memories, cache stores dynamically data/code which has been lately used in order to improve access speed, compared to standard memory connected to the global Avalon matrix. Every time a memory access is required, the processor checks if the required data is already present in the cache or must be newly fetched from memory; in the meantime, old unused cache data is being continously replaced with new data.

Tightly coupled memory is also a fast access memory, since it exploits a dedicated port, but it has static content: you decide what you need there and you specify it in the linker script.