All the suffixes after PAL, NTSC or SECAM relate to the way the signal is modulated and transmitted.
Some of the letters also indicate the field rate and Subcarrier frequency (i.e. the frequency used to modulate the color information)
PAL-M and PAL-N are ones which have different field rates and/or subcarrier from 'normal' PAL
PAL-B though is just a variation in the transmission details (e.g. Channel spacing, sound to vision carrier relationships and so on)
All countries have a specific letter for their transmission system and you would find that a TV set or VHS recorder designed for PAL-I (e.g. UK, South Africa, Hong Kong) wouldn't work in a PAL-M territory like Brazil - Probably you could tune in the sound or the picture but you wouldn't get both.
From my notes I believe that PAL-B,G (they are usually interchangeable) is used in most of Europe (excluding France and the UK) , Australia, Greenland & Iceland, India & S.E Asia, and a fair part of Africa.
Hope this helps!