Still need help with ereg

I am trying to create a validation script using ereg and I'm having some problems. I think part of this is due to not quite understanding what the purpose of magic quotes is - actually this whole idea things being escaped with slashes confuses me. Could someone explain in plain english what all this means. Also, what is the satandard - should get_magic_quotes be set on or off in the php.ini and what are the advantages or disadvantages. Thanks