PAL/SEACM and NTSC are two different television formats. These formats precede the introduction of the DVD. They actually even precede the introduction of the videocassette (Phillips produced the first VCR in 1972).
NTSC stands for "National Television Standards Committee" and was created in 1953. It is the standard format used for televisions in most of North and Central America, and Japan. In techno-speak it mandates 525 lines of resolution at 60 half frames per second.
PAL stands for "Phase Altering Line." Adopted in 1967, i t is the standard format used for televisions in most of the world (other than the US, Canada, and Japan). In techno-speak it mandates 625 lines of resolution at 50 half frames per second. PAL TVs are said to give a more consistant hue than NTSC TVs. Brazil uses PAL-M, which differs slightly (525 lines of resolution with 60 half frames per second). PAL is, for all intents and purposes, identical to SEACAM.
SEACAM stands for "Systeme Electronique Couleur Avec Memoire." Like PAL, it was adopted in 1967. The technical specification are the same as for PAL
Because PAL & NTSC are television formats, this issue is only relevant for DVD players or VCRs that hook up to a television set . Computer DVD players hooked up to computer display devices (which are commonly neither NTSC or PAL/SEACAM) will display the content of the DVD irrespective of PAL/SEACAM or NTSC format. The same goes for a VCR.
DVD/VCR players may be constructed to play on televisions that accommdate NTSC format, PAL format or on both. There is no legal restriction on producing or owning a player that plays on both TV formats. However, because televisions in a particular region tend only to be either PAL or NTSC, DVD/VCR players tend only to display on a single TV format. Go to Acquiring Videos, DVDs/VCDs & VHS/DVD Players to locate vendors of multi-format DVD/VCR players.