Supported Video Formats

FLV
Most played video format on the web. YouTube and Google Videos are using it.

Pros: Basically all clients with Flash capability can see these videos. Has been there since version 6.
Cons: Referred to a proprietary file format and does not offer as good video and audio quality as H.264 does.

H.264
A video compression standard.

Pros: Provides good video quality at lower bit rates than FLV. As a standard it is not Apple's proprietary format.
Cons: Not much.

MP4
A video compression standard that is a subset of the whole H.264 standard family

Pros: Same advantages as in H.264
Cons: There seems to be many different flavors of MP4 and only some of them are supported by Flash. You can always try a given file to see if it plays in the IVWP player, if it does not the most likely reason is that it is not compatible with Flash.

Notes:
H.264 isn’t really a video format. Instead, you’ll see it as an encoding option for some formats such as MOV (QuickTime). While a “regular” MOV file won’tplay in the IVWP video control, a MOV file that’s been encoded with H.264 will play.