First of all, if you are serious about actually learning what all of the different slots are in a computer then go get the book "Repairing and upgrading your PC". This is the ultimate Bible on anything hardware related to computers, and while the new version can be a bit pricey, you can get a used copy that is a few years old for $5 or less. A worthy reference guide to anyone who is looking to build or repair computers on a regular basis.
Beyond that there are 3 major card slot standards that have made their way over the last 10 years, and I will briefly go over them from oldest to newest with basic information. Look them up in Wikipedia for proper pictures and further details:
1) PCI
PCI has been around since the 1990's, and is so prevalent that even new motherboards tend to still have at least one PCI slot. These are designed for older/slower devices, including sound cards, network cards, USB/Firewire cards, and other things that add capability or connectivity to your computer, but don't necessarily require much in the way of power or throughput. On a normal PC you will find 32bit card slots, but there are much longer 64bit card slots that are found in servers and other high bandwidth applications. 64bit PCI then gave way to the PCI-X standard, which is often times confused with PCIe (or PCI Express), but in reality they don't have a whole lot to do with each other.
The next big player was AGP
AGP slots are similar in size to PCI slot (~4"), but tend to be the top most slot on the board and sit a bit further towards the middle of the motherboard, and is dedicated for graphics cards. This slot is now defunct, but was still used on occasion in some Dell D2400 machines. There are several versions of AGP including AGP, AGP2x, 4x, and 8x. Newer cards would fit in older slots, but would be choked on bandwidth, but older cards would not always fit in newer AGP standard slots. Just keep in mind that this is a fairly old standard that has not been used in a while as all newer GPUs now use either PCI or PCIe
Lastly is PCI Express
PCI Express is God's gift to nerd-kind! It has relatively little overhead with lots of throughput, and the length of the card determines the speed at which the card runs. At the front of the slot is a short section for power, followed by a series of connections for data. There are up to 16 repeating channels. Short cards that use only 2 channels are denoted as PCIe x2, while big graphics cards are PCIe x16. Other popular standards are x4 and x8, but technically card manufacturers can use any number of connectors that they want.
The nice thing about this layout is compatibility. If you have a short PCIe 1x or 2x device then it will fit in and work with a larger x4/x8/x16 slot just fine. Similarly, if you have a larger card like a GPU and you put it into a shorter slot (some x4 and x8 slots leave the back of the slot open for this exact use, while some overzealous modders will just cut a notch out of the back) then it will also work just fine, but simply run with the bandwidth available. This is why high-end motherboards simply have a crap ton of PCIe x16 slots on board, not all of them are physically wired to run at x16, but as you can fit anything anywhere it keeps things cleaner than having a wider variety of PCIe slot lengths.
Typically the top most slot is wired specifically to be your primary x16 slot which is intended for your GPU. 2-3 slots below that will typically be another x16 slot available for a secondary GPU which will typically only be wired for an x8 connection, which is still plenty fast for most GPUs to run with minimal bottlenecks. Other PCIe slots may only run at x2-x4 speeds and are meant for more modern sound cards, network cards, and other devices. Higher end motherboards will add extra PCIe controllers to expand the amount.
Lastly, there are (so far) 3 revisions of PCIe. PCIe1 was introduced essentially as a GPU alternative to AGP8x, and eventually gained support to replace AGP entirely. PCIe2 doubled the throughput, allowing higher bandwidth for high end GPUs. Up until this point card makers were still using plain 'ol PCI card slots, and were hesitant to move to PCIe because they would need expensive x4 controllers. But with PCIe2 many cards could run on a PCIe2 x2 slot just fine which allowed them to build physically smaller cards and use cheaper controllers. PCIe3 was launched ~2 years ago and doubled the PCIe bandwidth again, now allowing 1GB/s throughput per PCIe lane. Even 2 years later, this is an extreme amount of bandwidth, and I am not sure that there are any single GPU graphics cards that are actually capable of using more than 8 lanes of PCIe3. The standard was mainly moved to because PCIe2 was choking much larger cards that used 2 GPUs on the same card. Hopefully the single GPU cards released this year will finally make use of the bandwidth available.
Just as with the slot size compatability where the length of the card or slot determines the throughput, there is also extreme compatability on the side of PCIe revisions. You can put a PCIe3 x16 graphics card into a PCIe1 x16 slot and it will technically work... it will just be choked for bandwidth and may not have full performance. Or you can put an older PCIe1 x16 graphics card into a newer PCIe2 x16 slot, and it will also work just fine, but the card slot will revert to the older slower bandwidth standard.
PCIe is especially neat because it can be use for things beyond simple card slots. Thunderbolt (which I still wish was called Lightpeak) is an external data connector similar to USB which uses PCIe lanes as a way to transport data. This allows for the use of things like external GPUs, and other really neat things. Similarly mSATA (the smaller HDD connector found in some laptops and tablets) is based upon PCIe, and it's replacement called SATA M.2 is very much a small PCIe buss which can be adapted for a wide variety of devices manufacturers wanted to. On the desktop side of things it looks like SATA4 will be PCIe based as well which will further unify connectivity standards within the PC.
Hope that helps!
Caeden
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