PCI-e X16 slots are the largest ones and are used for video cards, for the most part. They can also be used for pci ssd's, but those are extremely expensive.
PCI-e X1 slots are the ones that are about the same length as the PCI-e X16, but don't have the clip to secure the card in and have a different slot altogether. These can be used for wireless cards and fan controllers and some people even get a usb 3.0 card that you plug in and it'll give you two extra ports to use. Also forgot to mention sound cards.
Finally, plain ol' PCI slots are the tiny slots on the motherboard and these are typically aren't used much anymore. They are Legacy ports that only get populated if somebody has some really old hardware. I haven't use any of mine and I don't see me ever doing it.
Hopefully that covers what you need to know.