how the data is transfered using PCIE LINK

I am using the altera S2GX for a project uses PCIE to transfer data.

Now we have the hardware working with the PC, since altera didn't provide the source code for the pc app.

I am not sure how the data is transferred to or from the fpga.

For example, what's BAR used for? if I am not using the DMA to transfer the data, is that mean I dont need the BAR?

I just want to do a simple read and write to the fpga, which address should i put the data on? where i specify the address i want to write to on fpga?

If you know any document talks about, please point me there.

Thanks