It all depends on what medium you plan to put your final work on; DVD, HD-DVD/Blue-Ray or Beta digital tape. I'll only explain the DVD form, since I know squat about the others.
With DVD, which is what I presume you'll be producing for home television viewing, there are only a few choices of pixel formats, 720x480 (D1 NTSC) or 720x576 (D1 PAL) are the only ones guaranteed to be usable on all DVD players. The others (480x480, 352x480, 480x576 and 352x576) may not play on older machines.
Notice I said nothing about aspect ratio? That's because it's not set by the pixels format, it's set by a few bits in the MPEG2 file and used by the DVD authoring program in other files. A DVD player uses this info to adjust how it converts the movie to the TV output, scaling the video and adding the top and bottom black space according to what the viewer has set on his player. 4/3 and 16/9 are the standard values, there may be others, but these are the most popular.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70