Generally speaking the main differences between these devices are:
Consult the documentation for your equipment or search the Internet to determine these two characteristics.

As far as the resolution is concerned, note that most devices can play videos designed for lower resolution devices without difficulty.

The device models available in ffDiaporama are as follows:
 
Device type Operating system Available resolution Remarks
Smartphone Nokia Symbian System S60 QVGA
VGA
Generally, Nokia phones working with the Symbian operating system and equipped with a camera, are capable of playing videos. Very often, those provided with a keyboard offer a 320x240 resolution (QVGA), while those with only a touch-sensitive screen offer a 640x360 resolution (VGA).
Movie format is: Container: MP4 - Video Codec: MPEG4 - Audio Codec: MP3
Note: Video files should be stored in the "My videos" folder after connecting the telephone to the PC in "mass storage" mode.
Windows Mobile 6.0/6.1/6.5 QVGA
HVGA
VGA
WVGA
Numerous manufacturers made phones using the Microsoft Windows Mobile 6 operating system (HP, HTC, etc.) between 2006 and 2010.
Movie format is: Container: 3GP - Video Codec: MPEG4 - Audio Codec: AMR
Note: Store video files in the "My videos" folder having connected the telephone to the PC in "mass storage" mode.
Windows Phone 7 WVGA Microsoft Windows Phone 7 is the evolution of Windows Mobil 6
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
Android
1.5/1.6
2.1/2.2/2.3
QVGA
HVGA
VGA
WVGA
Many manufacturers made or still make phones using the Google Android operating system.
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
Apple iOS iPhone 3G/3GS HVGA Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
iPhone 4 WVGA
HP - WebOs     WebOs arises from the repurchase of Palm by HP.
To be completed
Blackberry OS RIM 240 Movie format is: Container: AVI - Video Codec: MPEG4 - Audio Codec: MP3
Note: store video files in the \Blackberry\Video folder after connecting the telephone to the PC "mass storage" mode.
Samsung BADA     To be completed
Tablet Android 2.x
Android 3.x
Linux
800x480
854x480
WVGA Tablet examples: Archos 4/5/70/Arnova 10, Dell Streak, Creative ZiiO 7
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
1024x600 XGA Tablet examples: Archos 101, Samsung Galaxy Tab, HTC Flyer, Hannspree HANNSpad SN10T, Toshiba Folio 100
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
1024x768 Tablet examples: HP TouchPad
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
1280x800 720p Tablet examples: ASUS Eee Pad, Motorola Xoom, Packard Bell Liberty Tab, LG Optimus Pad, Archos G9
Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
Apple iOS iPad XGA Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
RIM BlackBerry Playbook 1024x600 Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
Netbook/NetPC Windows/XP 720p Require xVid or DivX codec to be installed on the computer.
Movie format is: Container: AVI - Video Codec: MPEG4 - Audio Codec: MP3
Windows/Vista 720p
Windows/7 720p Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC
Android/Linux 720p Movie format is: Container: MP4 - Video Codec: h264 - Audio Codec: AAC-LC