What does having less lanes REALLY mean?

I heard the new intel processor is going to have 16 lanes because it has to reserve 4 lanes. I also saw that the previous processors had 40 lanes... so my question is: What does having 16 lanes instead of 40 really mean?

I know that if you run two graphics cards at 8x that means you have 0 lanes left, does that mean all hard drives will run super slow? If I have one card at 8x does that mean I have 8 lanes left, and that means i can run a max of say 8 drives? What exactly uses lanes?

I want to build a very futureproof system and I'm thinking of going with the older 40 lane processor in case I'm going to need those lanes in the future...

Only the -E CPUs in the past had 40 lanes. Mainstream CPUs always had 16.

In practice, no difference.

Hard drives don't map to PCI-E lanes like that. Hard drives are connected to the PCH (platform controller hub), which is connected to the CPU via another bus called DMI. It is totally independent from PCI-E.

The PCH also has a few PCI-E lanes (that it will route through DMI), and they can be used for anything except GPUs.

If you have 1 GPU, it will use 16 lanes. If you have 2 GPUs, they will use 8 lanes each. In practice, the difference in performance is about 1%, because even the biggest modern GPUs are not anywhere near saturating PCI-E 3.0 x8.

In general, I would also NOT recommend building a futureproof system. It's much better to buy a $500 system every 4 years, than a $1000 system every 8 years, because of how fast computers become obsolete. The $500 system in 4 years will be much faster than the $1000 system today.

TL : DR
PCI-E lanes provide a link to the system
More lanes = more badwidth avalible
One the consumer side, aka NOT a server, you are never going to notice the difference bewteen 16 and 40 lanes under 99.99999% of the usage cases.

There are 16 CPU (fast) lanes and 20 "southbridge" (slightly slower) lanes provided by the chipset on the Z170 chipset. If you run SLI you will use 2 x 8 = 16 of the cpu lanes. All of the sata ports are already mapped to the other southbridge 20 anyways. Of course some of the mobo mfg's saw fit to add even more SATA ports with additional controllers that are slow relative to the chipset provided ones. So... I think its enough. Barely. PC's are never futureproof anyways no matter how hard you try. If you buy top of the line now, you may have little or no upgrade path in the future. The next series of Video cards might not even go into a PCIe slot. There may be one more rehash of Video cards coming but nothing spectacular before whamo, all this junk will hit the trash. I do not pretend to know for sure, Its just I never saw a 3 year old PC that had any useable parts in a new build. Other than perhaps the power supply and the DVD drive. Your definition of futureproof may be different then mine which just tries to run the flashiest games at the highest res possible. Perhaps you have some esoteric use for CPU based PCI lanes that I don't know but If you want Quad SLI with the highest overall bench scores, you need to go to X99 and a Hexa CPU with Quad SLI. If you can't afford that right now, rest assured that it will get blown away by some low end gaming PC in 3 years anyways.