Once you understand the basics, it's easiest to use more graphic representations of each one to understand them. I've also included a table at the end that details which color space is used for some standard video formats.
YUY2
YUY2 is a packed color format that uses 4:2:2 chroma subsampling. Each pixel has either a U or V sample, but not both.
YV12
YV12 is a planar color format that uses 4:2:0 chroma subsampling. Each chroma sample is located at the nexus of 4 pixels. In the diagram below the chroma layers are shown relative to pixel positions.
DV (NTSC)
NTSC DV doesn't have a name for its color format, but always uses the same packed 4:1:1 chroma subsampling. Some PAL DV formats (but not consumer grade DV equipment) also uses this subsampling. Notice that it uses the same number of chroma samples as YV12, but only subsamples horizontally, and locates both the U and V sample at the left most pixel in the group instead of centering like YV12 or locating the samples at different pixels like YUY2.
DV (PAL)
PAL DV, like its NTSC cousin, uses a packed color format that doesn't have its own name. Unlike NTSC DV, the chroma is subsampled at 4:2:0, but since it's a packed format the chroma samples are aligned to a specific pixel.
Version History
v1.0 2007.08.19 Original version by Rich Fiscus
v1.0.1 2007.08.19 minor edits by Rich Fiscus
v1.0.2 2007.11.16 Added links to additional DVF guides by Rich Fiscus