PAL and SECAM are totally different systems, but PAL is a slightly modified version of NTSC, which just swaps the phase of the luminance on alternate lines (That's what the acronym PAL stands for). The reason is to cancel out colour differences which NTSC was prone to have.

There are several different NTSC systems available, using a different number of lines on the screen. USA uses 525 lines and 60 pictures/second, I think, as opposed to UK/Europe with 625/50. Japan is different again.

This was all due to the US and Europe choosing different frequencies for their a/c power supply. In the old days, the TV frame rate (pictures/second) was locked to the mains, as the frequency was well regulated.

There are at least 8 versions of PAL worldwide, mainly differing in the sound subcarrier frequency, so you may get picture but no sound. UK is 5.5Mhz, Europe is 6Mhz, and it was soemtimes possible to retune the sound. Presumably the differences were specified in the old days to protect domestic equipment makers from foreign competition, but it backfired somewhat by limiting their export markets.