Jantar is the word from a common dinner. The dinner we have everyday.
On the other hand, ceia is something more special. Normally when we have a family gathering for an especial occasion (usually religious).
Ex: Ceia de natal = Christmas meal
Ceia do dia de ação de Graças = Thanksgiving meal
A última ceia = the last meal Jesus had with his apostles.
etc...
Hope I helped you a bit. If you have any further questions, just ask!