Ya what Ren said!
Here are some pictures to help you see the difference:
Video PCI Express x 16 (and most AGP slots) always have some form of locking mechanism at the end of the slot (see Pictures).
Also notice that for some unknown to me reason, Video PCI Express x 16 slots come in many colors (see Pictures).
The really small PCIe x 1 slots are for other things like sound cards.
The PCIe x 1 slot was originally designed for video cards, but most of today’s video cards require a PCIe x 16 (version 1.0, 2.0, or 2.1) slot. However, there are cards either on or coming on to the market that do or will make use of the PCIe x 1 slot. Sound cards, wireless cards, TV tuner cards etc. There aren't really all that many things using PCIe 1 x yet, but as regular PCI is phased out, more devices will be available. Currently Sound Cards, Network cards, and SATA cards seem to be the main thing PCIe x 1 is used for.
And to clear up some more potential confusion:
When talking about PCIe slots, you have to make sure the “x” description is what you think it is.
“# x” refers to the interface speed.
“x #” refers to the number of the specific slots.
EXAMPLE: My motherboard has 3 x PCIe x 16 version 2.0 slots.
This example would read; My motherboard has three PCI Express by 16 Interface Speed version 2.0 slots.
So I am able to put three PCIe x 16 video cards in my PC and run 3-Way SLI.
Hope this clarifies a few things for you.