What is the difference between PCIe Gen 3 and PCIe Gen 4?

Advanced Thermal Design & Thermal Guard：To Ensure VRM Power Stability & 25110 SSD Performance; EZ-Latch：PCIe x16 Slot with Quick Release Design

Hi, i recently bought a new computer and plan on getting a new graphics card, but the motherboard has two types of PCIe x16 slots,

pcie slot height this mobo has three pci-e slots physically 1x16lanes ,2x1line What i found, the x16 slot is supporting only one pcie line according to

pcie m2 slot PCIe x1 slots have only one PCIe lane Despite having only one PCIe lane, PCIe x1 slots are useful for various lower-power PCIe peripheral cards