PCIe slot on your motherboard is powered by the 24pin ATX cable from your PSU. It will power low power devices that don't need extra power like higher end graphics cards which require a 6 to 8 pin PCIe cable (or more based on model) from your PSU to the GPU, so yes, you will need to provide power to both.
The main 24 pin psu connector is what powers most motherboards, including a pcie graphics slot(max 75w)
Some motherboards are capable of running several pcie slots as well as heavy cpu overclocking.
For them, a extra 4/8 pin eps connector is required. Sometimes two.
If a graphics card needs more than the 75w that a pcie slot can deliver, it will need extra 6/8 pin pcie cables from the psu.
Typically motherboards that have extra power adaptors for PCIe slots are only used for extreme overclocking, for normal usage and even slight OC'ing you wouldn't need to plug them in.
My old Asus socket 1366 rampage 3 extreme had 1x24 pin, 2x8 esp pin, and 2 extra molex connectors on the board just for the PCIe slots. I always had them plugged in as i ran OC'd 2xGTX480, 2xGTX580, 4xGTX 670 superclock, and 4xGTX 780Ti classified