what's the difference between PCI and PCI express?

Answer 1

PCI is an old slot and bus, produced by intel in the pentium 1 era or around
the PCI bus was a 32 bit 33Mhz "Highway" where data travels. 32 bits means that 32 cars can travel one beside the other and 33Mhz would be the max speed of that Highway.
Doing some maths:
32 bit are 4 bytes ("user" is a 4 bytes word)
33Mhz means 33 million times a second.
So in conclusion, 4 bytes x 33 Million is the amount of data per seconds that can travel through the PCI bus, like 133Megabytes per second. That was a lot, compared to the even older ISA slot.

Then came the AGP bus and slot, physically different to the PCI slot, and faster and faster, it reached a rate of 2 gigabyte of data per second (that the AGP 8X one), perfect for video cards which really needs that fast highway

PCI express is the natural evolution, that is replacing both PCI and AGP bus and slot (PCI is still used today for small expansions boards, as 133MB per second are enough for a modem, an USB port, a sound card, etc)

The rate of the PCI express will depend on the flavor. We Have PCI express 1x, 2x, 4x, 8x, and 16x. they are physically different, but pin compatible, meaning that a 1X slot is a really small slot, but you can insert a 1X card into a 4X slot without problem. Not the same luck in the other way. You can't insert a 16X video card into a 4X slot due to its size of course.

Answer 2

PCI is an old expansion slot, that today takes the sound cards, network cards etc.

The PCI-E slot today is mainly the new expansion slot for Graphics cards.

Answer 3

7 letters e-x-p-r-e-s-s